I'm making a wallpaper for a series called "Zettai Shonen", but there isn't a section for it. Where should i do?

For now, upload it into the "Other" category. Since the series is still relatively new there isn't a category for it yet since there aren't enough gallery items. However, if in the future you notice there is enough items to warrent its own gallery entry, you can use a support inquiry and ask for the category to be created.

what Tama-Neko said