I always had the problem, that i could resize (drag with my mouse) images inside kupu, but when i save the document the image has its original size 🙁

Today i found the solution to this problem here

To enable kupu to keep the resize information upon save you have to add width and height to the style whitelist in the kupu preferences in your site preferences. Your styles whitelist should look like this:

text-align
list-style-type
float
width
height

that did the trick and now resizing images is working 🙂