That depends on the phone make and on the method of transfer you'd be interested in.
I'm assuming that the two of you are apart, so simple USB is out of question... That said...
I would resize the photos to a smaller resolution, 640x480 is pretty common. Use a light-weigh format, like .png or .jpeg (lower quality but you'll jam more in one message/send them for cheaper/send them quicker.
Once you've done that, you should put the photos on your phone. If you have an SD slot - use that. If you don't, use the standard data transfer cable which you recieved with your handset.
Next, send them via MMS... that said, wouldn't e-mailing them be easier? I mean, your girlfriend surely has access to a PC.