I've shared this site wherever I go because it's a great tool, a great place to start when you are choosing a palette from a single photo, and then they have great on-site apps where you can download or screenshot your creation. If you haven't used Big Huge Labs, go there now!
For those that are new and don't know what I mean by choosing a palette - this is the color scheme you can use if you are creating items to scrapbook your page with and you want them to match. I like doing this as a foundation then introducing other items of other colors.
If you are using Gimp, simply make a screenshot after you create your palette and then use the color picker in Gimp to transfer your colors to the color box. Note: you may want to save this screenshot for later reference.
If you don't know how to do a screenshot, here's how:
1. Adjust the item on the screen so you can see all of the content you want to copy.
2. Look in the upper right of the keyboard for a key titled "prt sc" or "prt scr". Note* My keyboard has a box around it so I have to hold down the function key "fn" and "prt sc" at the same time. Some keyboards you have to hold control "ctrl" and "prt sc" at the same time and rarely you don't need to hold down any key but "prt sc".
3. To test this out in Gimp, with Gimp open in the main toolbox window go to File> Acquire> From Clipboard. Note* When you do this, you'll see Gimp offers other screenshot options. I'm not going to go into detail on how to do those, I'll let you play with them on your own. I like this method since I often take a quick screenshot and dump it into MS Paint.
4. A window should have opened with your new image. If it didn't, chose a different key combination from Step #2.
5. Crop your image and make other alterations you wish and be sure to save.
Mac: [I do not use a Mac, never have.]
Go to About.com here for instructions.