Using Sigil with your AppImage made my problem with the display of some (but not all) gif images inside the InsertImageSVG plugin's dialog go away.
I couldn't believe it so I reversed and went back to using the /usr/bin/python3 path to the interpreter with the edit to /usr/local/bin/sigil wrapper script to remove the QTLIB_DIR statement. This made the problem with gif display come back.
I found that I could go back and forth between these two configurations and make the display issue disappear or reappear in a systematic fashion.
see :
https://www.mobileread.com/forums/sh...21#post4373721
So there's something in your AppImage that was missing or defective in the other setup configuration. How about that?