Here's what I'm trying and hoping I won't get false positives.
drop message = REJECTED - Graphic Attachment and bad HELO
\"$mime_filename\" HELO=$sender_helo_name
condition = ${if match
{${lc:$mime_filename}}{\N(\.gif|\.jpg|\.jpeg|\.png)$\N} {1}{0}}
condition = ${if match {$sender_helo_name} {\N\.\N} {no}{yes}}
drop message = REJECTED - Graphic Attachment and failed reverse
lookup \"$mime_filename\" IP: $sender_host_address
condition = ${if match
{${lc:$mime_filename}}{\N(\.gif|\.jpg|\.jpeg|\.png)$\N} {1}{0}}
condition = ${if eq{$sender_host_name}{}}
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