I came across a couple images (this and this) that weren't generating thumbnails that I tried fixing with a reupload and/or a rename, and nothing worked. I finally Googled it and found w:c:community:Thread:786656 where a Wikia staffer says that images over 3500 pixels won't generate a thumbnail. I'm pretty sure this is nonsense, so I'm gonna try to figure it out.
Can you make a shortlist of images you've uploaded that are over 3500 pixels either in width or height? Just a sampling to figure out what's going on, don't need a comprehensive list. Preferably some recent ones and some old ones (from before 2014) if you can find them.
Thanks for the heads-up! I noticed the problem occurring across Wikia for a couple of days now. I wonder though, is it an honest error that is to be fixed, or are they simply trying to discourage content creators from taking up too much space with (in their eyes) unnecessarily large images?
Another wiki I've found the problem to appear on is Memory Alpha, specifically on Star Trek: The Official Starships Collection (see the second image on the page, showcasing the "models re-scaled to in-universe dimensions").
Does that help? Let me know what you can find out; like if this is gonna be a thing we'll have to deal with for now!
Well, I submitted the bug to Wikia, but they're not staffed on weekends so I don't think I'll get a response until tomorrow evening.
I did reply to that thread I linked to though, and someone pointed me toward w:c:community:Thread:1431449 where it appears that they know about the issue.
So, I'm not gonna run around trying to find them all (I wouldn't even know how to do that), but it's something I'm keeping my eye on. I'll monitor some of the known offenders and let you know what I find.