I assume you might need to convert the files rather than just rename them?
When I had a similar requirement, I found an excellent Freeware program called IrfanView. This has many useful features for manipulating image files, including a batch converter which will convert all files in a folder from e.g. .jpg to .bmp formats.
IrfanView is widely available as a download from freeware library websites.
I hope this is of some help.