Trying to update my forum image, I'm told it's invalid with absolutely no explanation as to why.

I’m on mobile so I can’t see the image’s dimensions, but it’s possible they won’t allow you to upload pictures that are too large for the forum signature rules. Try resizing it to be a bit smaller and see if that fixes it

It's exactly 325 by 302, within the limitations listed on the page. I tried shrinking its width by a pixel, down to 324, just in case the stated maximum was off by one, but that hasn't helped.

If you're using the same image you have posted in the first post it may be because you're direct-linking from Toyhouse and not an image-hosting website. Places like Toyhouse are considered galleries, not image-hosts. You may want to try some place like imgur, photobucket, or stash on deviantart.

Can you find a staff post or something confirming toyhouse has been blacklisted?

With forum search down, no, but I know in the past I've personally had problems / seen others having problems with direct-linking things from Deviantart (not stash, but the main site) and it wouldn't work, the image would just break so that's why I'm suggesting finding an image host to trouble shoot. I could definitely be wrong, but figured it was worth the attempt to see if it's the image or the site.

Looks like you were right, I shoved the image into discord and it worked first try. I'll make a suggestion thread about getting more hosts whitelisted, but in the meantime this will do.

Also, DA doesn't work as an image host any more. DA themselves now maliciously break links. It's why I used TH, I couldn't think of any other reliable site.

I am not sure if Toyhouse can be whitelisted exactly... I'm not a coder or anything so I don't know, but I imagine the site may be more complicated than others because of authorization restrictions such as watermark / thumbnail / etc settings that can be given to each image. Not a bad idea to look into it though of course :) Glad that you got this issue sorted!