By default, user accounts do not have permission to submit image files to the site, whether those be screenshots or avatar images. 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 submit images using the appropriate forms.
To submit screenshots, 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.
To submit avatar images, first head to your "Settings" page on the main site. Click on the avatar currently in use to bring up a list of other avatars available. Scroll near the bottom of that page to find a link to the "avatar submission form" that you can use to submit your desired avatar. All avatars should be 100Wx100H pixels.
Once your screenshots or avatar images have been submitted, they will enter a queue and staff will decide whether or not they should be added to the site.