I personally like the camera and paperclip idea. But I’m biased because that’s what I’m doing…
@mcwumbly in up arrow’s defense: Should attaching an image be more obvious? - #5 by codinghorror
However the paperclip is overwhelmingly used (life according to Google Images) when you search “attach icon”
I think for a user’s understanding, “attach” is probably a better verb and related icon than “upload.”