Hi, I just wanted to let you know that in the case an existing art asset is updated (splash arts, champion squares, loading screens, etc) we do not update them until the content has actually made it to the live servers. If it is a new piece altogether, go ahead an upload it, but existing files shouldn't be updated until the content has been applied to the actual game. PBE changes can be tentative and might not make it into the game. For instance, Lux's splash wasn't actually updated in the live game until about a month after the initial intorduction to the PBE of the new version (after some alterations). A couple months ago, countless splashes were changed but only about 1/3 of those changes actually made it to the live client.
When you upload a file from the Special:Upload page, where it says licensing, you need to select one of the options from the drop-down list so that the file is categorized.
If you upload from any of the other ways to do it, or if none of the options in the drop-down are appropriate for your file you do it this second way:
Once the file is uploaded, visit the file's page and scroll down to the bar at the bottom that says "Add category". Click on it and type in the category you need. When you're typing, existing categories will appear below your text as suggestions.
If you're unsure about which category an image should be in, you can ask someone, or you can look at similar images to see where those go. For instance, if you're uploading new loading screens, search for existing loading screen files to see which category they're a part of.
In any case, don't be shy to ask for help when you're not sure or if you have any questions.