I have been having intermittent issues with my Moto Droid not sending MMS pix msgs, and it has only gotten worse since I've had my Droid, and I've finally found the culprit.
From one of the message boards on here I found an app called "Ultimate Juice" (which is the paid version of "Juice Defender"). This app does a fantastic job of saving tons of battery between charges, and "JuicePlotter" by the same author will estimate your battery use pretty accurately. I installed this app, and have enjoyed it profusely.
Today I finally was awake during the day, and had ample time to troubleshoot this issue with Verizon and actually was able to go in and have them look at my phone. After about 20 min of messing around with my phone, they gave up and I have a new Droid on the way. Upon entering my driveway I thought to myself, "Hmm I wonder if I ...." -- I uninstalled "Ultimate Juice/Juice Defender", deleted all my text message threads (had a few Pix msgs stuck still), and rebooted my phone. Voila. I've sent multiple Pix msgs and have had no problems. I reinstalled Juice Defender/Ultimate and still have had no problems.
If you guys have the same problem, try uninstalling your version and reinstall, delete your txt msg threads, and reboot the phone as normal.
There are also other other things that can be going on here as well. Here's another solution to try yourself if you are still having problems (found this while investigating my problem)....
How to fix the Motorola DROID MMS bug | Gear Live
Finally, good luck!
PS- I'm horrible at tagging, so please mods help me!