MMS'ing of pics, etc, in particular of Verizon to AT&T, is a very long and discussed issue. I currently have an Incredible and iPhone using both Verizon and AT&T respectively. Generally when I MMS something from iPhone to my DINC, the iPhone says failed but it will still come through. Now on the flipside, sending files from DINC to iPhone can either be rapid or, like last night, can take HOURS to finally come through to my iPhone.
Long story short: long term cross-network issue. Not the fault of any particular messaging app necessarily.