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One thing I learned a while ago was to use JPG instead of PNG as thumbnails on Leo. Seems like Leo doesn't like PNGs although this could fixed, don't know, haven't tested for a while.

Oh wow. Thank you for sharing your experience. It might indeed be the case as most of my thumbnails are in PNG format. I appreciate you :)

You're welcome! That at least sorted out the not visible thumbnails for me.

That's awesome then. I noticed that there are some images in other people's posts that are not viewable (displaying as "not found")