Allakhazam's has added the ability for users of the site to add their own screenshots to monsters, and zones.
For a more comprehensive guide to HOW to take Screenshots and use them in the wiki, see Bludwyng's A Photographer's Guide to Norrath.
Your screenshot needs to be in JPEG or PNG format. Often games will write out their screenshots in other formats (BMP for Everquest), but you can use a free image tool to convert your images in batches, and even do initial cropping and scaling. Illia's preference is XNView, though some people also suggest Irfanview or GIMP. Only .png images will preserve transparent backgrounds.
Your image needs to be a maximum of 1200 pixels in height or width in order to be accepted for upload.
When images are initially uploaded, they become visible to everyone, although they're flagged as new. Any logged in user may report an image in this state. If an image is reported, then that image and all images in the new state by the offending uploader are flagged as needing review. An image in this state is no longer shown on the page it was uploaded to.