When I switched from Windows to Ubuntu I had to find a solution that works on Ubuntu for Image editing so I went on to GIMP but unfortunately it was harder to learn and the interface looked really intimidating. Why not try LunaPic. I haven't used it so I don't know if it's useful but it looks like it can make transparent images.
GIMP is definitely essential. I use it myself, however the new release is pretty slow and harder to use in simple file updating. I recommend an older version, or if you're like me and you like to code on the go (GIMP installs DLL files so requires a custom build for this...) you can stick this version on your flash drive. http://portableapps.com/apps/graphics_pictures/gimp_portable A portable version of a great stable release of GIMP which runs quick from a USB.
fuzzy select + color to alpha + .png files = great sprite tool