Most graphics artist type work (logos, signs, fliers, etc.) are created with something more like Inkscape (Vector Art) rather than Gimp (pixels, photos). Both are free and pretty powerful, but all such tools take a lot to get good with them. Inkscape lets you draw the shapes, outlines, tweak edges (corners rounded, etc.) gradients, and they scale to any output size, etc.
Yeah, I feel your pain. When I first started out, I did everything myself, and learned quickly that in general it was surprisingly painful and frustrating. And likely whatever you do will still look amateurish. Bottom line is you could probably instead spend that time shopping around for some Odesk or something for someone who does that sort of thing cheap and likely on a fixed-bid basis if you can. Show them a mockup of what you want with as much instructive detail as practical (graphics can suck, but show everything you want, where roughly, etc.). Then get quote for it, like 3 samples to choose from, then they polish it. And be amazed at what you get back.
This has the advantage of ensuring you never get stuck doing it again, likely will start out more professional, and gets you adding contacts/delegating...two things you need to do for business. You should be finding ways to direct money to your profit. Not doing graphics art work. You don't need to create logos or signs or invoices, etc. beyond boiler plate stuff IMO, from my experience, and when you DO need those things, outsource it. Everyone does those sorts of home-grown things early on, and probably everyone I know that has, feels silly looking back.