It only seems to happen with .png’s, but when it does ALL .pngs are blank like this until a close the file-picker and restart it. If I’m lucky, it will recover.
Not sure this is a thing, but if that preview gadget relies on available thumbnails (inside the image, or external), perhaps none was created for the image…? Force it to create thumbnails on its own, maybe? I don’t know if that’s a thing one can decide either. Or maybe use something like ImageMagick to do that, yeah, extra work, I know.
Disclaimer: I don’t use that feature, hate the very idea of it. But that’s just me. I’m sure it’s a wonderful idea for those who like it.
As an aside, I personally use Gwenview, even thought it lacks heaps of features and its behavior leaves a few things to be desired. It’s pretty fast and easy to use. I also use Geeqie because of its image comparison feature and visual sorting feature (via its Collection window, drag’n’drop images around, then rename using a prefix), although it’s a tad crash prone if used in full screen mode. ImageMagick’s Convert and Mogrify to edit images in bulk. Heaps of fun you can have with those.
I think, this is because of Security !
Pictures have to be interpreted to create a thumbnail or preview. If there is a vulnerability in one of the image libraries, looking into a preview is all it takes to exploit it. 