You should be able to display base64 encoded images directly in a web viewer, using =
"data:image/gif;base64," & Table::Base64Data
as the address.
I just tried a test, you are right it does work. I did however run into a diferent problem when working with my sample data. It seems that the image is encoded rotated horizontaly. Filemaker does not offer the ability to rotate the web viewer window. The Decoded GIF is also much larger then what I need.
I am thinking of taking the Base64 data and adding html and css code to it to rotate the image, and scale it to the size I need. Export the field data to text file and load the text file in the web viewer. I would do this real time as the layout with the image on it is loaded. Even though there would be preformance issues, the base64 image needs to be refreshed every time the record is assessed since it may change. Although this will work, I am not sure if it is the best solution.
I haven't tried this myself, but if it's possible to rotate and scale the image using HTML/CSS, it should be possible to do it directly in the web viewer using a calculated data URL - with no need to export.