Ok, one last try to install this widget. Everything seems ok except the actual image not showing up. The browser instead shows the regular question mark that means a broken image link.
Have given the whole cache directory and its content all permissions (777) while trying to get this to work, to no avail.
Can it have something to do with the fact that cached images are named something like 65816ef48316b6ce88c882c8506be097.spc, whereas the html asks for an image called "rss_image_cache_8"? It seems to me that's the transformation moment that doesn't happen.