Pictures on SMS is called MMS, and MMS requires that your cellular data be turned on and you are charged for data, unlike SMS which is not charged. XM has a size limit of roughly 1/2 MB for an MMS. This applies whether you are using the cellular network or WiFi calling.
For advanced messaging see the recent article on RCS for Android phones.
Textra now has a setting under MMS that temporarily turns Wi-Fi off when you send MMS messages so that they will go out on LTE, specifically due to this limitation with Xfinity and other carriers.
I found this note on Textra
XM/Verizon does not support MMS over WiFi. You must have Mobile Data ON to send / receive MMS.
DONNOT USE TEXTRA, OR SIMILAR APPS, ON XFINITY MOBILE
If Textra works at all, it turns on cellular data and turns off WiFi and is unlikely to turn WiFi back on, which means that you could inadvertently use a whole lot of data.