I recently had the above question on an image on my Facebook page and I think it's a great one. I certainly remember wondering how some people managed to get such super looking images when mine always seemed a bit lacklustre. As with most things technical, there isn't a single answer to this, but these are some of the principles I've learnt that improve image clarity.
There are 3 main areas:
I'm going to break each area down a bit and discuss separately.
1. Capture the best quality image possible
Raw not JPG
It goes without saying that in general the better quality the camera you use, the cleaner and better image quality you can expect, but I think it's wrong to assume you can't create clear, sharp images with any modern dSLR or compact camera. Sure, if you're going to be blowing them up to a huge size then you'll need a medium format camera or similar to get the resolution needed, but here I'm talking about web or average size print output.
Whatever camera you use, you need to make sure you capture the most data possible with each shot. Where possible, this means shooting in Raw format, if your camera has the option. In short, Raw format keeps all the information from the camera at each capture, whereas .jpg modes discard about 80% of the data, after the camera has made it's decisions about how the image should look. Having as much data as possible from a capture means you have more to play with later if you wish to optimise or creatively edit it in an image editing program such as Photoshop. I'm going to use the term 'Photoshop' for image editing program from here on, just because that's what I use.
If you don't have Raw, then shoot the highest quality and resolution mode possible. And next time you upgrade, look for a camera with Raw support - even many compacts have it now.
Sure, you are going to get sharper, clearer images from professional-range lenses. But this is most important if you aren't going to be editing them afterwards and they're destined for immediate use (e.g. using .jpg capture mode). If you edit your images in Photoshop then you can improve them to nearly match the sharpness and contrast that comes out-of-camera with more expensive lenses. Expensive lenses offer you more than just better sharpness and contrast, so it's not the whole story of course.
Correct exposure (or slight overexposure)
A correctly exposed image always looks better than one you've had to 'rescue' in Photoshop. Getting accurately exposed images comes with time and experience shooting. An accurately exposed image is one where there is a full range of brightness levels in the file at the time of capture. The 'range' that you need will depend on the style and creative decisions you make for your picture. It is very helpful to understand histograms when learning about exposure. Figure 1 shows the histogram of an image that is underexposed. There is no information at the right or highlight end of the curve.
Figure 1 - underexposed
A properly exposed image generally has an even spread of the graph from left (dark tones) to right (bright tones). This is a slight oversimplification, but let's run with it for now for the sake of this discussion.
Digital camera sensors capture a whole range of information about your images - details of structure, light, shadow and colour. It is very important to understand that the most information is captured in the brightest stop of the histogram. Not just information about brightness, but all the other stuff too.
The second brightest stop captures only half of all the information of the first and so on (Figure 2). Most digital SLRs can capture 8 or 9 stops of light in total.
In Figure 3, I have overlayed the other two graphs onto teach other. As you can see, by not capturing any information in the bright areas, we are effectively not using a huge proportion of the camera's sensor, and reducing greatly the amount of information available when it comes to processing later.
This means that when I make exposure decisions at the time of taking a photo, I generally aim to slightly over-expose an image, just to make sure that I capture as much as possible in the highlight end of the histogram. Importantly however you have to do this without blowing out the highlights. 'Blowing the highlights' means that areas of your image are so over-exposed that you have nothing but white, with no detail at all. In this situation, no attempts to pull back information later are going to work - although Raw files are much more forgiving of this than .jpg captures. Of course the image will look to bright when you start to edit it, but that's easily fixed with a couple of sliders and the overall image quality will be improved.
2. Editing techniques to preserve integrity
This includes things such as:
At the time of editing you will use various techniques to improve the image such as increasing contrast, saturation or vibrance, local sharpening and/or clarity adjustments. This is an extension of your artistic vision that began at the time of taking the photograph.
3. Optimise images for desired use (output)
Images look best when they have been prepared for their final use. This means
If you are going to load images onto the web, you need to know the final size they are going to be viewed at, because they will always look better if YOU do the size conversion (and before the final sharpening step). The same goes for images destined for print.
For Facebook, for example, I know that images will only be shown at a size of maximum 2048 pixels on the long side. Therefore, I re-size and then sharpen the image myself before uploading, because I know the quality will be better if Photoshop does it, than if I feed full resolution, unsharpened images into Facebook's upload tool. Images larger than 100kB will be resized by Facebook. In practice Facebook's image algorithm is awful and it's hopeless to think you can consistently present high quality images via this website. It's worth noting that Facebook pages have photos resized to 1MB, so 10x less compression. And some believe that saving files as PNG results in better quality - I've done several tests of this and been unable to tell the difference.
Sharpening is a topic in itself, on which whole books have been written. Essentially, there are 3 stages to sharpening an image. Most dSLR images need slight sharpening before you start editing them - capture sharpening. This applies to Raw images especially - camera .jpg files will have some sharpening applied depending on your menu settings. Capture sharpening can be applied via a Raw converter or in an image editing program. You can then apply creative sharpening to the image in Photoshop - e.g. the eyes of people or other areas that you want to 'pop'. Creative sharpening is optional and most images don't need much if any at all. Finally, all images - especially after resizing - need output sharpening. The amount of output sharpening depends on the final use - less for web, more for print.
Adobe Lightroom takes care of output resizing and sharpening for you in one step using the Export command. Other software does this too, but if you are finalising an image in Photoshop, you'll have to resize, then sharpen and finally export this Facebook version if you want the best quality file to upload.
I hope this helps explain some of the ways you can get a better quality image from what your camera delivers. Shout if you have any questions!