The best way to upload screenshots and files is to create an album first.
Once you have done that, head to your profile and click the My Gallery tab under your avatar/canvas. You should see Albums you've created appear underneath:
Click on one of your Albums that you'd like to upload things into, and you'll see more details on that Album, and be able to upload items directly into it using the "New Upload / Link" button:
You can upload screenshots or files directly into Albums, or you can link to content somewhere else on the web.
- Images are limited to 1920x1080 resolution, and no larger than 1MB each (JPG with 90% or less quality are recommended).
- Files are limited to 10MB size, and need to be one of the following types: zip, rar, doc, pdf, txt, xls, 7z, sims3pack, package
It isn't absolutely required that you create an Album before uploading screenshots or files - you can upload them directly into your Gallery and move them into/between Albums you make at any time by editing them (the small pencil icon in their upper-right).