Yes, thanks. I knew about the path but was having problems with an already created user bank. The problem is that the referenced graphics (at bank creation time) do not get copied over. They only get mentioned in the *.dat.
My advice: never, ever modify the dat files with an editor (even if it might look like plain text). You might loose hours of work.
Regarding the size: I would recommend someting between 250 and 500 pixel. 125 pixels are fine for the library preview but if you click on your bank/preset in the library tab the graphic might look terrible.