I created a own Webitem. Everything works perfect.
I registrated it in all the necessary tables (RSRRENDERER...).
Now I want that it has its specific preview image in the WAD. I uploaded a GIF to the MIME-Repository and entered the right values in RSRRENDERER (like the SAP objects).
The strange thing is, when I enter a SAP image it works perfectly.
Do I have to consider something special for the file format?