For ImageButtons you can use PNG, BMP, JPG files perhaps even other formats.
For the transparency, the system set all pixels having the same color as the upper left pixel Pixel(0,0) transparent. See the ImageButton's help file.
I am using BMP files, I tried JPG files but had trouble with those because surfaces with the same color (transparent color) didn't have this color any more and parts of the surface were no more transparent. I've never used PNG files.
Best regards.