The first thing you want to do is get the best picture you can so you don't have to do too much editing.

Since your using a P&S (I'm assuming) try and get the composition you want from the camera so you don't have to crop anything, also Sixbolt was right about the location, just take it to an empty parking lot/garage or something. Make sure you have the camera set up to take the biggest, highest quality image it can and maybe do some exposure bracketing since you wont really be able to fix the exposre in post (unless your camera shoots raw).

As far as editing goes I would play with the contrast a bit, maybe some sharpening (very little), tweak the colors if you need. In the case of your sig I cropped, de-saturated the color, bumped the contrast, sharpened, balanced out the exposure as much as I could, then added a gradient map just for kicks and of course the text.

I use Paint.NET btw, which imo is much easier to use and learn on than GIMP. It might not have as many features but it has enough to get the job done.