Every time I try to set a forum image, it says the image is invalid. The image is within size range and I used imgur, so I can't figure out what the problem is. Is there something else I need to check?

If the ending is gif, it won't work. Subeta's gone anti-gifs in forum images, they still work in the signatures though. Try a jpg, jpeg or png.

They've all been images, no gifs.

- Did you pick the correct url to use? The direct link url is the one you need. Anything else won't work.

Yep. After that didn't work for the hundredth time I started trying the others too, but obviously that didn't get me anywhere. Looks like it has to be a Subeta issue...

The image also has to be hosted on a site that supports https, such as imgur.

That's what I've been using. Finally got it to work though! Apparently imgur had trashed some of their options, or at least they weren't showing up on my tablet.