This rule shows the "do nothing" pattern, which was added in the v1.6 preview.
# A "do nothing" rule, using the special replacement pattern (-). This is most useful when
# used in front of a series of other rules. This one says - for any URL request that ends
# in a .jpg, and includes no question marks, do not rewrite it.
RewriteRule ^/[^\?]+\.jpg$ - [U,L]