If you are a little more adept at using photoshop, you might try Gimp-shop, it is based on the gimp, but made to look and act more like photoshop, in fact almost an exact replica from what I hear.
To draw a straight line, btw, you use the pencil, hold shift (if I recall correctly, suppose it could be control or alt), click where you want to start and then click where you want to end.