I printed my name (first and last), signed it and wrote my ERAS number. They had no problem accepting it. If you are a foreign grad you need to attach a special sheet with it, I'm not sure if you need to do the same for US grads.
If you are an IMG, the ECFMG instructs the following -
"reference your name and AAMC Identification Number on the reverse side of photo, or the photo will be discarded"