Whenever images are included in Wikipedia, it makes a big difference if they look good. When they do, an article appears more professional and is more pleasant to read. When they look amateurish, the article looks amateurish.
Images can often be improved – or even transformed – by using graphics software such as Photoshop or the free GIMP and Paint.NET applications. Many other programs also have photo-enhancing tools, including facilities for semi-automatic image enhancement, so that you need only click a button, or choose a thumbnail, to have a positive effect on image quality.
Always upload modified images under a new filename, and link between them and the original.
Here are some things that should be kept in mind before uploading images:
It's also possible to make seemingly impossible corrections, such as correcting "leaning" buildings or removing unwanted obstructions. Perspective distortion can be fixed afterwards, or a tourist standing in front of an otherwise clearly-photographed building can be cloned out. This can be difficult to do well, even for experienced image editors. If you clearly see that the image needs improvement but feel that any of the above improvements are best left to an expert, upload it anyway and then ask for assistance on Wikipedia:Graphic Lab, the help desk, on this article talk page, or elsewhere. You'll find a ready supply of willing editors capable of first rate improvements – all you have to do is ask!
English Wikipedia allows images to be hosted here which are public domain in the United States, but not their source country. With the exception of these (which are forbidden there), remember to always upload images to Wikimedia Commons, the repository for all metawiki images, as the image is then made available to projects in other languages. The wikilinks to images are exactly the same as those uploaded directly to en:Wikipedia. If you make any of the above improvements to an image, upload it under a new filename, and link between it and the original. You might also want to submit your images to Picture peer review, where it will be critically assessed by experienced contributors and – who knows? – might be considered a suitable featured picture candidate!