I've only used private key steganography and only on a limited basis. As I understand it, the thing not to do is use an image that's freely available as the "tinkered with" image can be compared with the base image to detect changes in the image data. I've never tried steganography with public key encryption.
My favorite applications at this time are GPG, Tor and Torbirdy (amongst a few others).
You'd have to use your own artwork, obviously :-)
You can also use GPG to create a DES/AES encrypted message. Of course, then you have the issue with conveying the key.
Public key would be better, but with modern keysizes, you'd need decent sized images to be able to hold all the data you'd need. The best thing to use with stego would be video or audio. A small bit of video could hold a lot of information. Again, you'd have to source your own stuff though, because you wouldn't want someone to be able to compare against an original. The internet has made it brain-dead stupidly easy to arrange dead drops and such.
I hope it makes the slime at the NSA stay awake nights, just thinking about what we could be doing right underneath their noses with our cat videos.