There's SMS and then there's MMS. Because these differ you need to have your carrier check out the APN for MMS use. You've proven the SMS is OK but with pictures, well, that's an issue across carriers and sometimes the iPhone has it's own way.
Back to the carrier to check it all out.
So I have a Galaxy S6 that I have had since release date. It's the 128gb version on ATT. I don't think I usually have problems contacting anyone in particular, my phone always makes calls correctly, seems to always send texts etc.
I have a good friend that I text all of the time that has an iPhone 5s. Whenever our phones are next to each other there's never a messaging issue it seems. I have tried everything in my knowledge to fix this issue. Reseting, clearing all messages from both phones, making sure my message center # is correct, etc.
Now for the issue,
When using Emojis it seems most of the time, or with picture messages the phones will sometimes "not" receive the message from another. I'll reply to a message from the iPhone and it will never ever get it even though it's sent on my phone. Or the iPhone will send me a message and even though it's sent on the iPhone I never get it. These are often long text messages with Emojis so they're not just a standard text message. If anyone could help me figure this issue out it would be MUCH appreciated. It wasn't a problem when I first started contacting this phone, but it gets worse and worse almost to the point I cant contact it for more than a few messages and then they wont get my message or I wont get theirs. The issue seems to be the iPhone not getting the reply from my S6 more often for sure.