One way to do it is this:
/path/to/images/dir/.htaccess
# If the URI is an image then we allow accesses
SetEnvIfNoCase Request_URI "\\.(gif|jpe?g|png|bmp)$" let_me_in
Order Deny,Allow
Deny from All
# Allow accesses only if an images was requested
Allow from env=let_me_in
Andrea