There may not be a need for anything more than a simple REN command. See How does the Windows RENAME command interpret wildcards?.
As long as none of your file names have multiple dots in the name, then the following should work for the current directory. Just make sure there are at least as many ?
as the longest name.
ren *.jpg ??????????????????????????????????????1.jpg
Or to process the entire directory tree
for /r %F in (.) do @ren "%F\*.jpg" ??????????????????????????????????????1.jpg
Or you can iterate each file with a FOR or FOR /R loop and rename them one at a time as foxidrive does in his answer.