Hello again, sorry for the late reply. Too many other emergencies happening...
I am still having the same problem although I am using a new laptop since December. I made other colleagues try - it worked without any problem for one of them for the other one it also does not work (tested in Chrome and Firefox) so I don't think it could be a general rule from our IT guys. (In the meanwhile the "insert image" is also not working in Firefox any longer as it seems…)
I can also add images for example as thumbnails or as the icon for the widget which I assume is more or less the same function (isn't it?).
Any other idea where to look at?
I use TrendMicro OfficeScan but it doesn't show any activity and from the settings I don't find anything about blocking actions.