You cannot "convert" older .rem files from Blackberry phones back to original JPEG pictures (.jpg, .jpeg) files with some kind of converter.
You will have to disable the encryption on your BlackBerry device in order to remove the rem suffix from your files to get back the unencrypted originals. For this you will have to install use the BlackBerry desktop software and of course you will also need to know the encryption key.Open REM file Open JPEG file
RIM application used to synchronize of BlackBerry smart phones with PCs
Mac version of software for synchronization of BlackBerry smart phones