Zooming along. Rated 5 out of 5 stars

hurdingkatz may have had the same issue I did.
I found this extension by searching "image zoom".

I was unaware that Thunderbird's default Zoom setting is "Text Only". Once I unselected that, images zoom as well.
However, the icon isn't available if you open messages in a new window. It's fine in New Tab or if using the Message Pane Layout, neither of which fit my use.

I'm specifically looking for an extension like "Image Zoom" where I can simply right click an image and pick a zoom level without zooming the text.

Other than that, this extension works fine!

This review is for a previous version of the add-on (0.0.6). 

Text Only?

Thanks for the rating.

I would really like for this to work for you.
Zoom Button simply provides one click access to Thunderbird's default zoom feature including full page zoom, and text zoom.
It does not specifically zoom images, it zooms all content in a message. If there is an image in the message, then it should also increase in size when you zoom in.

Please be sure that you do not have Thunderbird's Zoom setting set to "Zoom Text Only" or images will not be zoomed.
View (menu) > Zoom

The screenshots at the following address shows an image zoomed in a message loaded in a tab.
If you still have problems, please let me know what O.S. you're using.