I have the same probleme with Teams, I on Pop!_OS 20.04 (base on Ubuntu 20.04).
If you you Right-Click on the image and then Left-Click on the image, it will work.
Images do often not open in Teams at Ubutnu
I am using Teams desktop version on Ubuntu.
Almost always if someone sends me an image in chat I try to open it but I just see a blank screen, even the chat is gone. This happens all the time:
What I try in order to see the image, is that I open another chat and go back to old chat, so that I can see the chat again. Then I click on the image again. Often I have to repeat this 4-7 times until I actually see the image. Its really not usable like this. It even got so bad, that people have to send me an email with the image, because it will simply not show in Teams.
Any idea how to fix it please?