This tweak is probably common knowledge, but i discovered this for myself ages ago.
Note: This tweak works with Windows 7.
The Picture Puzzle sidebar gadget looks okay, but sometimes you may get a little bored of the default images that come with it. A Star, a fish whatever. They are dull.
So what can you do?
Simply save one of your favourite images (that would work well with the picture puzzle gadget...or at least look good ) at the dimensions of 108x108 to your Desktop as a .png file.
Now go to C:\Program Files\Windows Sidebar\Gadgets\PicturePuzzle.Gadget\Images
(your driver letter may vary) - look for the image you least like in the folder, and take ownership of it by editing the permissions. Make a note of its name (4.png for example) and rename yours appropriately so that the Picture Puzzle gadget will find it.
Agree to Copy and Replace, accept any other messages that appear.
If you should get an error there are two possibilities.
1) you have failed to take ownership/gain full access to the file
2) the picture puzzle gadget is running, if it is...close it till your finished. You can leave the sidebar itself on.