I use GIMP, it's open source and can do pretty much anything Photoshop can do. You can download and install plug-ins, new paintbrushes and stuff. If there's anything I want to learn how to do with GIMP, I just search for how-to guides on Youtube.
I also went from knowing nothing about graphic design to at least knowing how to make some pretty decent looking thumbnails.
I've done all my graphics and thumbnails with GIMP, it's for PC & Mac and it's free.
Thumbnail examples:
Header for my Twitter Page: