How are you attaching the photo? Are you going to your gallery app, selecting the photo, then Sharing it using Textra? Or are you in Textra first, tap the attachment button, and then select the photo?
If it's the latter, are you selecting the Google Photos option, or something else like Gallery, Camera, or Files?
You can go to photos.google.com on your desktop browser to see if your photos are in fact getting backed up there.