I had reset my camera image counter to test out a new firmware and wanted to restore my camera's image counter back to 1720ish instead of 0001. I found the threads below on how to do it; here's an overview:

1. Delete everything off your card or format it
2. Take a photo and remove the card from your camera
3. If you want your next image taken to be img_1725.jpg, rename the folder to 117canon and the file inside to img_1724.jpg. Notice the highlighted numbers; these are the ones you use to rename the folders based on the "hundreds".
4. Put the card back into your camera and take a picture. The next one should be img_1725.jpg.

DP Review: Re: How to change the starting number of image number sequence #1
DP Review: Re: How to change the starting number of image number sequence #2