There is now a Chrome extension that's much easier to use than Fiddler. It's called "YoWorld Art". Once it is installed, you can open yoworld.com in Chrome, and double-click it in the extension bar at #3 in the following sequence:
1. Resize pictures to 390x260
2. Post your image online somewhere. (Post on Imgur, right-click and select "Open Image in New Tab". Copy that link.)
3. Open your paint board, if not yet open.
4. Open YoWorld Art extension.
5. Paste your picture link into the text box.
6. Enable the redirect option at the top.
7. Click the "Set new image" button.
8. Save your paint board on YoWorld.
9. Turn off the enable redirect option at the top.
10. Open and re-save the paint board on YoWorld.
The last two are very important. If you don't do that on any image(s), the next time you repeat steps 4-6, those images will change to whatever you just pasted in.