Use layers, in the dialogs menu hit layers, when you select an image you can have it start off transparent, but either way, you should draw on a transparent layer, then when all the layers are done, they do exactly what you expect, they layer. Delete the background image and it will be transparent wherever you didn't put something
Layers are also really great for trying to do something a few different ways, like a border, you might have 3-4 different borders and with the layers tool you can click the little eye (to hide layers) and compare the borders. same thing with text and images.
Layers are also used in animation when you save as a .gif you can select save as animation, then each layer is a frame in the animation.
Gimp has some great stuff if you know where it is. I found that about an hour of checking stuff out, (looking at effects, seeing where options are ect.) that it is extremely easy for me to swich from photoshop to the gimp, and that the gimp is favored over photoshop because I am more familiar with it. Once you are familiar the rest is just what you can imagine, or if you are lazy the rest is whatever you feel like making.
Anymore questions there are lots of people here that know gimp and photoshop and image filetypes and such to help you.
