Hello,
I think the most widely used way is by plotting the results with Python. Using it you have complete control over the final image. Within the Telemac installation directory there is a folder called "scripts". There you can get the scripts to manipulate selafin files.
For Matlab there is also an utility for reading and writing selafin files. As well as with python, you can manipulate the figures as you wish. It's but a matter of choice.
Other than that you can use basically any programming interface that has a graphics system. You only have to read the selafin there and write the code that will save you images as you want.
Hope this helps.
Best regards,
Phelype