The point is that phone service and access to the outside internet is blocked. You can't get images out any more than you can get text out, so putting text in images does not help.
If data centers can make outside connections then steganography and internal (to Iran) routing can help get messages from protests to someone that can pass those through datacenter Internet links.
Just being able to organize internally is super useful. The protestors don't need to use Facebook to organize effectively.
