BILLKENDRICK  Little children aren't supposed to worry about where files are saved. :^) It is, however, documented in the README documentation. Additionally, the Tux Paint Config tool lets you override where files are saved, if you'd like (it can be useful in networked labs at schools, for example).

Finally, to open previously-saved images, just click "Open", and a dialog will appear showing thumbnails of all previous works!

PS - I have my own Mac now (thanks to the folks at icculus.org), so I'll be able to dedicate more time to the Mac port.  
(Version 0.9.15)

BILL KENDRICK  Good luck! ;^) You might, however, want to just use "Tux Paint Config," a GUI configuration tool that lets you set things like window size, full screen mode, sound on/off, and override which folder images are saved in.

See: http://www.newbreedsoftware.com/tuxpaint/download/macosx/ for the link, of course! Enjoy!  
(Version 0.9.14)

BILL KENDRICK  Anonymous wrote: "It doesn't tell you where the files are saved"

Actually, the README documentation tells you. ;^)

"allow you to name them or open old ones."

Naming files was not something I had planned originally, since the program was meant to be as simple as possible. However, a number of teachers and parents have asked for this capability, so it's planned for a future version.

As for opening old pictures, just click "Open!" Duh! ;^)

"The stamper only stamps little penguins and the colors are limited."

You obviously didn't install the Stamps collection. It's a separate download, as it weighs in at about 2x the size of the program itself. It currently contains about 200 pieces of imagery, some photographic, some drawn. As this collection grows, it will be split into separate downloads (e.g., "coins" will be separate from "fruit" will be separate from "planets"), and Tux Paint will at that point provide a method of navigating the various collections (rather than lumping them all together, like it does now).

Enjoy!  
(Version 0.9.14)

BILL KENDRICK  FYI, the 'crash on Open' bug that got introduced in 0.9.11 was fixed in 0.9.12. Sorry about that! ;^)  
(Version 0.9.12)