The zip command has an --exclude (or -x) flag to exclude some files:
zip -r --exclude 'img/' compressed_filename.zip path/foldername
Adjust the paths if necessary.
You can also use the find command to list all the files to be included and pass them to the zip command.
find path/foldername -name 'img' -prune -o -exec zip compressed_filename.zip {} +
This will search the path/foldername for all files (and folders). If it finds img, it stops processing it (-prune). All other (-o) found items will be (together, because of + at the end) passed to the zip invocation.