Tap a picture in the chat to enter the Textra SMS preview mode (you can swipe between all photo's for a chat in that mode) then you'll see options like Share, Forward etc and tap on the 3 dots to see more options (as per the screenshot below).
Or,
You may go to the intended chat > tap on the expand icon on the top right = please tap on Media to view all media in that chat (as per the screenshot below).
