Hello mmc472,
Welcome to Apple Support Communities!
I understand you want to know how to enlarge a picture that is sent to you in Messages. I'd like to help and provide you with the information you need to enlarge the picture.
Typically, when you’re in Messages and have received a photo, tap it once to open it to full-screen. This is the screen where you can also zoom in on the photo. But, if this is not working for you then try the following.
Make sure your phone is up to date.
Update your iPhone, iPad, or iPod touch
I would also recommend restarting your phone to see if that corrects issue.
Restart your iPhone
If you're still having an issue then the next step would be to contact Apple Support directly.
Get Support
Let me know if that worked for you.
Thank you for using Apple Support Communities.
Take care!