Small proposal: add another <if> to postbit_attachmentimage, to remove caption tooltip when description is empty.
Also, I added title parameter as it is more standards compliant than ALT (i.e. tooltip will work in Mozilla/Firefox, too).
Code:
###########################################################################################################################
# postbit_attachmentimage
###########################################################################################################################
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FIND:
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alt=""
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REPLACE WITH:
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alt="<if condition="!empty($attachment[description])">$vbphrase[attachdescription_caption] $attachment[description]" title="$vbphrase[attachdescription_caption] $attachment[description]</if>"