By default, user accounts do not have permission to submit screenshot image files to the site. This is a measure to prevent image spam and server strain. If you would like to contribute resources for other contributors to use, you'll need to contact site staff to request additional account privileges. If your account is approved, you can then submit images using the appropriate form.
To submit screenshots once your account is approved, simply navigate to the profile for the relevant game while signed into your account. Then click the "Submit relevant image assets" link in the list of available options that appears below the game's information box and any image gallery already available. On the page that appears, you can submit as many as 10 images at once. Please familiarize yourself with all image guidelines posted on that submission form. In general, images should be 640 pixels wide. Do not submit images that are wider than 640 pixels, as they may not display properly on future revisions to the site.
Once your screenshots have been submitted, they will enter a queue and staff will decide whether or not they should be added to the site.