I would suspect a problem either between your phone and the network or their phone and their network. Since it is your phone that is continually sending, believing it has not succeeded, then most likely your phone and the network. Try rebooting your phone then retrying ?
Just because your phone is capable of sending photo attachments doesn't necessarily mean your network profile is so enabled. Check with your Service Provider that "MMS" capability is enabled for your profile.
I had a similar problem recently in a photo I was trying to receive - it turned out to be too big for my phone to cope with. Photos use up a huge amount of data. I turn off the data function when I'm out of the UK.