First of all: thank you very much !!!
whatever you meant by "make those li items render"

... I tried here, I tried there and finally choosed to go the radical way and installed everything new (this time your new beta by the way). This solved all minor layout questions. Interestingly the "-" I had already removed them before from the templates but it did not work. Very strange
Now, with the new install all these problems are gone. First IE 7 showed up strange behaviour with the image sizes by displaying the images in different sizes (even though all pictures actually do have the same size of 600 x 371) or by repeating part of the pictures to give them their original size of 600 width. This issue I could solve by adding the &disable_js ... to the snippet call.
Still remaining with me is this strange problem with the white pixels - due to your advice I deactivated all flash components in the site template as well as all unneccessary stuff in my yaml content.css - nothing
I even went back to the original photo and controlled if there are any transparent or white pixels (there are none, of course) and saved them anew as jpg, thinking maybe something went wrong during the batch picture processing - nothing
I updated my IE - nothing
As far as I know

I do only have the two scripts "motools" and "drop-down-menu" running in the site template
Deactivating mootools in the site template - no transparency layer, just a wonderful big picture in the left top corner but without white pixels
Deactivating also the &disable_js_libs=`1` in the snippet call: transparency layer
and white pixels
same results with deactivating drop-down-menu
Maybe IE has problems with cutting out correctly the transparency mask, biting off too much pixels
I do not understand how all this stuff works, I just imagine it’s doing something like this ...