ilbmtoppm < pic.ilbm | pnmtopng - > pic.png
To batch it you can use something like:
First have a script file ilbmtopng:
#! /bin/sh
echo processing $1...
ilbmtoppm < $1 | pnmtopng - > $1.png
(Remeber to chmod u+x it)
Then use it like this:
find somedir \( -iname '*.iff' -o -iname '*.ilbm' \) -type f -print0 | xargs -0 -r -n1 ./ilbmtopng
NOTE: If you don't have netpbm package you can install it with:
sudo apt-get install netpbm