Hey everyone -
Good news: pictures and attachements already support read only!! i.e. if there it is read-only, then there is no link to upload or change the picture / attachment.
Bad news: somewhere, in some screen cleaning up, the "read only" checkbox has been set to hidden....so you can't even set that property the code already supports.
That's clearly a mistake on our side, and a very simple fix (i.e. changing one place in code from "false" to "true").
On the next publish to the January version (Friday or Monday), you'll see a read only checkbox if you edit an attachment and click Advanced.
Pretty sure this is exactly what you are asking for.
Paul