the signature images are generated in the "jdegre" site when you upload a new set of skill values. once it is generated, the image is a normal PNG image file, exactly the same as any other static image on the web. so, it's not that the images are dynamically generated upon request of the PNG image file, or anything like that.
after some more investigation, I think that the issue might be much harder than I initially thought.
the main issue is that, apparently, the forum software takes a snapshot of the external image only at the moment when you crate your signature. then it stores such snapshot locally and it uses that image from now on, to display the signature in forum threads.
so, when i said before that the signature images are not "dynamic", what i meant is that they are not generated "on the fly" when the image is requested by the browser; in that sense, they are totally static image files, hosted in an external site (jdegre.net).
however, given how the forum software works, the image is only read by PCF once: at the moment of creating the profile signature.
if that is the only available working mode for the new PCF software, then there is nothing I can do from my side to fix that. it's simply that the forum software does not support the concept of hotlinking to external image files, and it only uses the image content once, when the signature is created.
EDIT:
I'm unfamiliar with Xenforo sofware, used by PCF, but doing a quick search on the web, I found that the forum can be configured to refresh the images it keeps in cache at certain intervals; I don't know how this is currently set in the forum but, if the forum would be configured to refresh cached images, say, every day, that could be an acceptable setting to refresh signature files, I guess.