By default, anything posted on Steepshot shows up on Steemit, they are the same blockchain.
I believe Steepshot is using tags to filter the content they display to only the relevant photo content created in their app, but I'd think it would be quite possible to enable crossposting images from Steemit to Steepshot. It may be as easy as adding a specific tag, or the Steemit team could add a share button or checkbox to the posting UI. I'm happy to see Zurvanic is already reaching out to the Steepshot team.