Vignetting is where the light distribution isn't even from edges to center. It generally means your light-gatherer (the lens) isn't a perfect match with your light-recorder (the sensor of the camera). It's most likely to happen when the hole the light goes through (the aperture) is at its largest (lowest number; for your lenses, f/3.5 and f/4.5). This is not limited to cheap lenses; I've got a 16-35 f/2.8 that cost me $1500 used and wide-open it vignettes on a 35mm frame size. Granted, enough people griped about this that Canon remade the lens within the year. And you can usually deal with this sort of thing in post. I don't know how stretched your finances are at this point, what with the camera and all, but please god start to use something more friendly than Gimp. You're now investing in your own skills and your own skills are heavily reliant on the tools they're trained in. Don't invest in software that nobody with any self-respect uses.