I’m sorry but I feel the need to call this out because it feels like a blatant contradiction.
I don’t agree that it’s currently hidden, in your case you’ve opted to expand the functionality of uploads, so Discourse has intelligently changed the icon to reflect this.
Agreed, some services do provide a little image button to upload images, particularly those which exclusively offer the ability to upload images, which Discourse did by default and you moved away from. Still, user choice and all that.
Wait, what?
Your initial assertion was that Discourse doesn’t make uploads obvious enough, although I think we can agree from the above that it does until you change the default behaviour. Your criticism was that uploads weren’t clear, but again until the default is changed it’s at least equal to that of other services. You’ve insisted that you need to keep other uploads enabled, but are now looking to conceal that functionality behind an image icon, effectively making that option much less obvious?
Don’t get me wrong, you’re entitled to change it to any icon you want. Heck, you can use a 
if you so wish, but isn’t it a little bizarre to make everything else less obvious instead of engaging in a tiny bit of user education?