I mean. When I use Synfig, I paint using the program, but I'm asking if I can paint using other program (Photoshop) and then animate the image whith Synfig.
Yes, it is possible. Just be sure that when you export the image from the raster editor you don't export the background (so a transparent background is exported). Also you need to export the image to a format that supports transparency (png, tiff, etc). Jpeg doesn't support transparency.
Once you have the images saved and imported to Synfig Studio (File->Import) you can play with them in several ways. The most complex one is shown in this tutorial:http://www.synfig.org/wiki/Doc:Cut-out_Animation
Once imported the images in Synfig you cannot morph from one to other with interpolation, like you would do with the vector shapes you could draw inside Synfig Studio.
Hope that helps.
PS: Welcome to Synfig forum!
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