Reads like you need to remove the background and that depends on your images.
Take the 20 png's and File -> Open-As-Layers As a check go to Image -> Mode and check RGB
edit: If you previous optimised the animation, un-optimise it Filters -> Animation -> un-optimise
Then it depends on the image layers but say the background is white you need to add an Alpha Channel to each layer Layer -> Transparency -> Add Alpha Channel This is a bit tedious so I attach a plugin anitools.py that bulk adds.
Next to remove all white Colors -> Color-to-Alpha for each layer. White is the default. If your paper colour is black, color pick that. Another, a script this time c2a-all-layers.scm
screenshots: https://i.imgur.com/TEYnDrp.jpg
Once that is done, export as a gif animation. One frame per layer (replace) for all frames. Check the animation. Note not all gif players correctly display the transparency mask as a chequer pattern, some white, some black.
screenshots: https://i.imgur.com/bamVTvP.jpg
Take the 20 png's and File -> Open-As-Layers As a check go to Image -> Mode and check RGB
edit: If you previous optimised the animation, un-optimise it Filters -> Animation -> un-optimise
Then it depends on the image layers but say the background is white you need to add an Alpha Channel to each layer Layer -> Transparency -> Add Alpha Channel This is a bit tedious so I attach a plugin anitools.py that bulk adds.
Next to remove all white Colors -> Color-to-Alpha for each layer. White is the default. If your paper colour is black, color pick that. Another, a script this time c2a-all-layers.scm
screenshots: https://i.imgur.com/TEYnDrp.jpg
Once that is done, export as a gif animation. One frame per layer (replace) for all frames. Check the animation. Note not all gif players correctly display the transparency mask as a chequer pattern, some white, some black.
screenshots: https://i.imgur.com/bamVTvP.jpg