Transparency in the Gimp is called "alpha". If you go in the Layers window, and right click the layer and Add Alpha Channel, it will add the ability for that layer to show transparency. Then if you select something with the selection tool and CTRL+X, or Edit+Cut, or use the Eraser tool, it will leave transparency behind. Before adding an alpha channel, it will use the current background color for those things, instead of giving you transparency.
If you're making a new image, you can select the background to be transparent by default, instead of "Background Color" or whatever the default is. Then you don't need to add an alpha channel by hand.
When you save the file and are ready to use it, save it in a format that supports transparency. Your best bet is PNG depending on what you want to use the image for. GIF supports transparency, but not full alpha-transparency (so far as I know); GIF is either transparent or not. JPEG doesn't support transparency at all.
Alpha-transparency (which PNG supports) lets a pixel be, say, greenish blue AND 50% transparent so it tints the background greenish-blue when you view the image on top of another image. Note that Internet Explorer is a broken piece of crap, and can't display images in this format, in case you were thinking of using this for a web site.
If you're going to save the image and work on it again later, then Gimp's native XCF format will let you pick up working from where you left off. Saving in other formats may force you to flatten the file into one layer (PNG for example), or destroy the transparency / reduce the color depth (GIF will probably do this), or reduce the image quality due to lossy compression (JPEG). |