I don't know about that, there seems to be more problems then just the programs themselves. I archive those images in my Deviantart's Sta.sh and look at this:

One image is okay, but the rest displays a "PNG No Preview" icon especially when it's visited:

Even when I download them from my Sta.sh, they still posses such error. It pretty much indicates that weird image-redirect-to-page-if-you're-not-logged-in-Fandom that was introduced created some sort of strange "modification" to their format somehow.

Especially the fact when I don't do "right-click and save image as" and simply drag the image to my desktop, it becomes a "Chrome HTML Document" instead of a .png or .jpg file. Other images that aren't from Fandom that I drag to my desktop at least kept the same file format they are from.

Where did you get the images you uploaded to your stash? If you uploaded them after downloading them the same way, then it could be an issue with how your are downloading them. What device are you using and do you have this issue with all images on Fandom or just some?

The same exact ones I've linked to. My computer is a laptop that's Windows 10 operated, and I've also remember downloading an image from the Animal Jam Wiki, also suffers from this, but interestingly enough, I've tested out downloading images from the Rocket Knight Wiki and it seems to be fine when they're opened in Paint.NET and Paint SAI.

Well, if you got your stash files the same way from the same source, then I wouldn't be surprised that they don't work either. The image that worked before, if you try to download it again, the exact same way as with these images, does it still work?

Redownload the same file I've linked up, still brings out an error in Paint.NET and Paint SAI.

I really don't understand why those images from that wiki suffers this.

And this is the file from Rocket Knight Wiki that you said was working before?

The Rocket Knight Wiki images have no problem, the files I've mentioned in the previous post is from the Rayman and Claus' Skylanders Fanon Wiki.

For the ones that work, are you downloading them the exact same way as the ones that don't (browser, drag vs. right-click, etc.)? I tried the dragging method on my Windows 10 using Edge and it saves as a WEBP when I do that. Perhaps that is what you are getting.

I download the images from the Rocket Knight Wiki via "right-click and save image as" cause dragging it, will result the HTML document.

Okay. And if you download the images from Skylanders Fanon wiki using the right-click, what happens?