The term "path" can be misleading to people who haven't worked with UNIX systems before. When talking about the path to a program in the UNIX world, this means the directory AND the program name. If you just give the directory, 4images won't know which program it has to run to make the thumbnail.
So, when you supply this:
/usr/share/ImageMagick
4images tries to run a program called ImageMagick from the /usr/share directory. Since ImageMagick is actually a directory, this produces an error.
Try this instead:
/usr/share/ImageMagick/convert