body:png only returns emails containing embedded PNG images. Is there way to search for any image format?
the only practical method is to search for messages with Not Has Attachments and filter by size - emails with embedded images will typically be larger than 100 kB - less than that will only find emails with embedded signature/footer images.
which would be
size:large hasattachment:false according to https://lookeen.com/blog/tips-to-narrow-your-search-criteria-using-the-most-important-search-commands.