Using an unlocked Pixel 2. I can send photo messages no problem, just cannot receive them. At&t carrier. Not an iMessage issue because I haven't had an iPhone in 5 years.
Most likely it is because iOS 11 has a default photo format that is High Efficiency Image File Format (HEIC) that is not compatible with Android or Windows.
If you want to be able to receive photos from your iPhone friends, you will either have to ask them to convert the HEIC images to JPEG, before they send them to you, or you will need ask them to turn off the HEIC photo option on their iPhone.
Below I included screenshots with instructions for both...