Hello Jerome.Giles,
We understand your mother-in-law uses iMessage to send picture messages but this uses data from your cellular plan. We'd be happy to provide information.
While there isn't an option to only use iMessage while on Wi-Fi, if unlimited messaging is available on your plan, you can disable iMessage to be used and enable it when the device is connected to Wi-Fi. You can find information on the difference between iMessage and SMS/MMS message in the following resource to get an understanding of how they're each billed through your carrier: What is the difference between iMessage and SMS/MMS?
Additionally, Apple loves to hear how customers wish to use their products. Let your voice be heard through Feedback. Go here to share your thoughts: Product Feedback
Thanks for using Apple Support Communities. Have a good one!