It's okay, here's a quick step-by-step process for you:
1. At the front of our page, click on the brown "Gallery" tab located at the top, 2nd from the "Home" tab
2. On the right-hand side of the Gallery page, click on the tab "+ Submit to this Gallery"; A drop-down menu will appear, so click on the tab "Contribute an Existing Deviation..."
3. A separate page will appear, showing your deviations in your gallery (in this case, your cosplay photos). Select a photo, and then click on the tab next to "Submitting to", labeled as "Featured"; A drop-down menu will appear, showing the gallery folder names for each category. Select a category that best fits your photo, and finish off your submission by clicking on the green "Submit" button, located at the bottom of the page. (See our Rules and Guidelines for categorizing photos in their respective game titles, located at the front of our page)
4. Repeat Step 3 if you wish to submit more photos into our gallery. Afterwards, click on "Done" to finish your various submissions.
Hope this helps!