So you're making this fabulous creation and you now want to save your work, but it won't let you, you get an error. You ask yourself "why can't I save an image with Gimp?" and you get frustrated. Well, Gimp is a little different than Photoshop when it comes to saving. It gives you drop down options of where to save but not how to save. The answer is really simple: when you save, you also have to save the extension. If you want it as a jpeg, you save as title.jpg, as a png you save as title.png, same for gif, psd (photoshop document), or xcf (gimp version of a psd), etc. Simple but not obvious. Does that help? Yay!! LOL =)
UPDATE June 2012: Most of my tuts are made with Gimp 2.2 in mind but now in 2012 we are up to 2.7, things have changed a bit. Gimp now, inconveniently, saves as a default of .xcf, so in order to save as a .jpg or any other extension you need to go to File> Export. If it's already the file you need and you just want to overwrite, choose overwrite. If your save was successful the info on the top left of your image box will read as Untitled (exported), if it didn't save it will still say "imported". Until you are used to this method, have the folder open where you are saving your images to make sure it appears before closing the image. When you close it will say you have unsaved changes, even after you have "exported", simply because you didn't save it as .xcf. I prefer 2.2 and have managed to get many new tools to work. I do have 2.7 on another computer. Goes to show you, keep a back up copy of older executable files in case you want to revert back to a different version. There is a site called "Older Version" where you can get these files. Good luck and happy gimping!