I have an animated gif with transparent background, and I want to add a background image to all layers:
How can I achieve it without adding it to every layer individually?
In the GIMP it is possible with the multi-layers-merger extension from here:
(archived project) https://github.com/pixlsus/registry.gimp.org_static
(location of plugin extension within that project) https://github.com/pixlsus/registry.gimp.org_static/tree/master/registry.gimp.org/files/
Install the extension, then
Create a new image the same size as your animation, and put your background image in a single layer in the new image.
Go to Layers > Multi Layers Merger
Deselect all the options but "Use only active layer". This means to only merge the active layer of the background image.
Make sure the "Second Image to merge with" dropdown is the image with your animation frames in it
Click OK, and a new image should be created with your background behind each frame.