I'm on ATT and have the same issue, but did come up with a very interesting clue which may be useful to someone. My wife has an iphone 5s on ATT and I switched to a MotoX from an iphone. I am un registered from imessage so that isn't a problem. Everything works great except some picture text messages from iphone users can't be viewed on my MotoX phone without a work around.
But, here is the really interesting part. My wife realized that every time from her Iphone that she types a text message and includes a picture inside of the same text to me, it won't display properly on my phone or in my stock messaging app(without a work around). But, if she types the text in one message, then sends a separate message with no text but only the picture then it comes through just fine every time and I can view the picture.
This is 100% repeatable. If she sends me a picture with no text included I can open it no problem. But, if my wife or my sister send me a iphone text message that includes text and a picture in one combined text I can not open the picture attachment without a work around. So if you tell your friends and family to send the text and picture attachments separately this problem should go away for you.
My wife knows about this problem so she sends them to me correctly all the time. My sister doesn't know about this, so for her I use the work around. I simply save the picture attachment, use the ES File explorer, and go to the 'download' folder. Then I can see a thumbnail in ES File explorer of the attachment, but can only open it full size with some third party apps. I use the ES Image Browser to view the file. At least I have a work around that works and know what the root cause is. But, I don't see why the stock Google Photos app or Gallery app can't view these types of jpeg attachments but third party apps can view them.
I know there is a lot of finger pointing going on, between apple and google, but since third party apps can view the attachment I feel like this is a google problem and not an Apple problem. If Google would update their photo app to view these type of jpeg files (like the ES Image Browser does) then it would work fine on Android. It must be in some special kind of jpeg encoding that is used when the text and image are combined and the google stock apps just need to be updated to view that type of encoding.