Rock? No. Bomb? Yes. You have to "insert" a bomb into the back of the canon. After you have aimed it correctly, go around to the back of it and place a bomb inside the canon. This should trigger the canon to fire and hopefully, if you've aimed it right, will blow up the thing in front of it.
Sorry. I messed up. You need to put in the canon ball first and then a bomb. I don't think you can put in a walking bomb but there are bombs available in the house if you look carefully enough. If you cannot find any regular bombs then you must leave the house and buy some because you cannot beat the dungon without them. Here is a helpful hint though:
Spoiler Alert!In the room where the Yeti is making soup, if you walk past him to the northwest corner you will find in one of the vases the oocoo bird so that you may warp out of the house, buy bombs, and warp back in.