i made a tool that creates the EXTRA.BG.PVR file from a pvr texture,
or directly from a png/gif/jpeg image.
in the case you choose the direct conversion keep in mind the biggest
the dimensions are, the heaviest the resulting file is.
here are the images dimensions and the corresponding size:

16x16 = 512bytes ~= 3 blocks
32x32 = 2048bytes ~= 7 blocks
64x64 = 8192bytes ~= 18 blocks
128x128 = 32768bytes ~= 66 blocks

im not sure you can use intermediaire dimensions, but what is sure is
you cant get higher than 234x234 (almost 200 blocks).
the texture is automatically readjusted anyway so even if you choose a
64x64 image the result will look good.
it has been tested with dk3 and mozilla but should work with any dc/pc
browser.

http://bswirl.stormnetwork.net/bioshack/extrabg.php