okey, let's see. what is meaning with image?
How exactly did you install RetroPie ? Did you use the file provided at retropie.org.uk/download (.img.gz) or other methods/files ?
The usb part i've configure to read roms trough the usb. I followed the steps where i first add some roms to the sd card, then transfered them to the usb. i don't know if you understand what i mean, but i hope you do.
Use the Finder to transfer your files, over the network, and leave the USB alone for now - see the file transfer method using file shares.