iMessage and picture problems

I am on a chain with 4 other people and we send texts and occasionally pics back and forth. Lately we have been getting a lot of "not delivered" messages when trying to send a pic. It seems that some of the people will still get the pic, but others won't and there's no rhyme or reason, but if the pic is sent as a text, it goes through fine. I tried to start a new thread, but that doesn't change anything. Is this a known problem?