I deleted some Sub-Devices via "Groups -> MyNetwork -> ....",
because I didn't need them anymore.
However, the images (shown as a broken symbol in many browsers) still remain in the shown website after clicking on my server in the device-tree.
How can I remove them?
Click on the "edit" link at the bottom of the page with broken images. This should show you a list of graphs and allow you to delete those that no longer have sub-devices.