>>48592167I use GIMP to make them, they're just gifs that have one frame for 100ms, then switch almost immediately to the next frame and don't loop. (should be shown on the bottom right of the program)
You make two layers, one on the bottom with your thumbnail image with "(100ms)" listed at the end of the layer's name, and another above it with your resulting image and "(100000000ms)" at the end of the layer's name. The layer names tell it the timing for the gif's frame and while it's not meant to ever loop, the resulting image's layer has a long timer just in case. The bottom frame is the start of the "animation" and the top frame is the end.
After that you use File > Export As... and change the filename to have ".gif" at the end. Export it with "As animation" ticked and "Loop forever" unticked, can't remember if you need to change the frame disposal setting or not but (replace) works.
Also as a sidenote, if you keep the GIMP window up but just close the tab showing your image in the program, it'll keep your export settings (the last part of making these) so you can make these in quick succession.
https://files.catbox.moe/1bv714.png 
