So to say: thank you MODXpo! There I first stumbled upon this promising Snippet.
I installed the Version 1.1.1 on MODX 2.2.10 and everything went fine.
But: either I use it as a snippet-call or as output modifier, there are no files generated.
I tried it on my local MAMP installation and on a hosted server.
GD is installed, no error is shown (nor in the MODX error log nor in the php.log).
Does anybody of you facing the same problem?
Thanks for your help.
fabian