I have a personal image with a grey background, I want to change the background color to white.
I have Pixelmator, but other solutions are welcome as well.
To do this, you can use the Magic Wand tool:
After selecting the tool, click and hold on where the grey background is, and move the cursor to increase/decrease the tolerance. Make sure the tolerance is just right, and covers the whole of the background, but not reaching the picture:
Once the area you want is selected, select the Paint Bucket tool. You then select the colour that you want from the swatches, which is white (if you dont see it press ⌘ Command+6). Now click the image with the paint bucket and viola, a white background!
Save, Quit. Done.