I guess it is simple...but I have no idea how to solve.
I do have a folder with about 1000 pictures inside. About 600 of those pictures are used within 4 InDesign projects.
How can I find out which pictures are not used in oe of the 4 projects and could be deleted?
I am using InDesign 5.5
You could package the files. In the package are then only images collected which are used in the layout (File > Package...).
a script could used here.