Keep in mind that "buttercream" covers an awful lot of area, ranging from the simple cold-process buttercream that's been on the back of the C&H powdered sugar box since before most of us were born (my choice), to complex recipes involving eggs, simple syrup, and lots of whipping.
Also, keep in mind that there's no law that says you have to practice on real cakes.
Last April, when I was trying to work out the right consistency for writing on the nearly-vertical sides of an unfrosted Bundt pound cake (my father's birthday cake), I used the heels of a loaf of bread: they were a fair approximation of an unfrosted cake crust.
If I were a complete beginner, and needed to get to know and love a piping bag, I'd do what all the experts seem to be saying: practice on an inverted cake pan, then scrape it off, and put it back in the piping bag.
If you want to try edible printing, I suggest you find someone who does it, and sells unmounted images, and that you start with an image that's less than a full sheet, so you can have it printed at least 2-up on the sheet. This gives you a spare, in case you have to scrape it off and start over. It's not just for photographs; anything that's too complicated or too precise to hand-pipe is a good candidate.