|I assume you mean from a command prompt?|
If so you will need to direct the path to the folder containing the jpg file.
The syntax I would normally use for that particular file is:
Ren "I G_0009.JPG" "9.JPG" so the syntax including the path would be:
Ren "C:\top folder\sub folder\I G_0009.JPG" "9.JPG"
Another way is to run the short expression as a batch file from within the folder containing "I G_0009.JPG". That way eliminates the need for stating the path.
Always pop back and let us know the outcome - thanks