I’ve got a sequence of pictures which were rendered out with 25fps.
Now is there a way in blender (VSE or Compositor) to remap to 24fps, so the duration of the sequence stays the same but with only 24 pictures per second?
I know I can easily remap keyframed animation with the Old and New settings in the render panel, but this doesn’t seem to work with a picture sequence out of the VSE or Compositor.
Thanks for any input
Sorry. I misread your post at first. My original reply was the “Old and New” settings. But, you already knew that. 
Blender on.y deals in discreet frames from external sources. Sadly this means that you will have to drop a frame per second to retain the duration, Blender does not have a frame blend function or optical flow estimation. Apply a speed effect strip to the source and shrink it to fit the required duration. It will look a bit staccato I’m afraid.
In general this is not something you want to do. You can add a stretch/compress strip in the vse, if you need to combine shots with different fps, but this generally works quite terrible and wreaks havoc with your audio sync. If you really insist on doing this, you should probably do the conversion outside of blender. But as this uses optical flow, you might get some artifacts.