Same disappointment here. I often am texted photos of important documents that I need to deal with for my job. With every other text app I've used (many) I can long press on the image, touch the Share icon, and choose to send the image to my work email account as a Gmail attachment. From the received email in my work email account I can easily download & file the image, or star the email for followup, etc.

To repeat -- how it should work:
- long press on image
- choose email/gmail app and send the image

With Textra:
- save the image to phone gallery
- go to Home screen
- go to Gallery
- press image and share via email
- delete image from Gallery
- return to Textra