I haven't done a great deal with Tkinter either, but when I was messing around with it making this:http://python-forum.org/viewtopic.php?f=12&t=1040
I did find that it seemed hard to maintain constant framerates but as said, my experience with it is limited.
If what you are wanting at the moment is just to create some simple 2-dimensional games then I would also suggest pygame. If you have a good grasp of python basics (including functions and classes) you should be able to pick it up at a fair pace. You also might want to look into pyglet. From what I hear pyglet is all-around slightly better but I haven't messed with it myself. It also supplies OpenGL bindings out of the box if/when you decide you want/need them. You can use OpenGL with pygame as well but it requires using the module pyOpenGL as well.
I can't really comment on which would be easier for you to pick up. I'm very comfortable with pygame so I haven't put in the work with pyglet yet. I'm sure if you stick with Tkinter you will probably be able to accomplish what you are aiming for; it just probably isn't the best way to go about it.