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If you can't afford classes, I, sadly, do not have suggestions outside of taking a cue from blanka and/or trying Pixelmator.
UCLA Extension has some great sounding design courses but all are at least $550-700 a pop (They are on my online university of choice for non degree coursework.) NYU is a little cheaper, but their online network is a headache to negotiate. New School and Parsons are even more expensive than UCLA.
If you dig a bit online you might be able to find cheaper alternatives, but with funds being tight it's tricky.
Pixelmator is a very cheap and fantastic Photoshop substitute (presuming you're using a Mac.) $29.99 at the Mac App store. Updates have been free for some time too.