2. To represent in a drawing or painting, especially in perspective.
7. Computer Science. To convert (graphics) from a file into visual form, as on a video display.
These would be the two definitions I deem appliciable for this case, and #7 is by far the most common usage of the word when dealing with computer graphics. Which we most likely are now, as he used a computer program to make that signature.
I'm not commenting on the quality of his work, just what I think is inappropriate usage of the verb or noun "render".
The signature in its entiriry could be called a render though, if you feel like stretching it
.