Image format and sizes for board squares

[Old Silverback]

What image formats and sizes need to be provided for a board square. iPad requires .PNG, some of the earlier versions used .BMP.
Is there a single format that works for all platforms? In other words, is .PNG good enough for everywhere, or do I need both .PNG and
.BMP?

Also, what image sizes? I seem to remember 137x137 for the iPad, but on "action pack" that I downloaded had a 77x77 .GIF, which I
converted to a 77x77 .PNG and it displays OK on my iPad.

Old Silverback