I am unable to use Textra on my phone after sending four txt smogs with any images even. It says safety buffer issue. No data left. Upgrade or you will be charged. I'm monthly customer for Tesco.
Sending texts of 140 characters or less goes as a text, unless you have Textra set to allow breaking texts up. (Then it will send a longer text as individual 140 character texts.)
But sending a picture, no matter how small (or sending any file, no matter how small) uses MMS, not SMS, and that goes via data. (You can always use wifi instead of mobile data until the next time your account rolls over.)