403 Forbidden Response is seen for Images when using Dispatcher, a / is also appended to the end of the filename in the URL.
The extra slash is due to Apache's mod_dir module which appends / to a request URL that maps to a directory.
Set DirectorySlash directive to off in Apache's mod_dir module:
<Directory />
<IfModule disp_apache2.c>
# enable dispatcher for ALL request if this is too restrictive,
# move it to another location
# RequestHeader add uid "admin"
DirectorySlash Off
SetHandler dispatcher-handler
</IfModule>
Options FollowSymLinks
AllowOverride None
</Directory>