Fine, taht works great, also my admin-preview works with that (doesn't work with the standards).

Is there a possibility to conserve the ratio of a picture and have the with and height as a maximum?

Example:
ratio_height = heigt / height_max
ratio_width = width / width_max
ratio_new = max(ratio_width,ratio_heigth)
height = height / ratio
width = width / ratio

How can I define an invidible background-color?
Thanks a lot for your help,
Christian