Make sure your using the most up to date version of Handcent (I believe it is 2.9.21) because I know they were doing alot of work on MMS capabilities.
Also, I was having general MMS "issues" when I first picked up the Droid but I found that by updating all of my mobile numbers in my address book to the 1-312-555-1212 format my messaging issues were solved. I would take out the parentheses on the contact you want to message, like the format I described above, reset the phone and then try sending the photo again. If it works, update all of your contacts.
I updated mine by exporting a google.csv file from my gmail contacts. I did a "file - replace" using excel and then reimported my google contacts into gmail which then updated my Droid OTA. I reset the phone and Voila!