@ODZ To answer your questions on this subject;
1) I use GIMP to edit these sprites, occasionally with Inkscape if I need to make a custom mask.
2) You can get these sprites by downloading the SuperMechs APK but beware, you will need adobe flash or illustrator in order to convert them from a vector (.svg) format to a bitmap/raster (.png) format, doing so with other programs have ended in corrupted or messy images for me. I get @Fluffeh to extract them for me when the APK gets updated as he has the software.
Sorry for the late response, I'm not as active on the forums as I used to be.