PSP X4 sertainly does not support the ancient LBM / IFF Amiga picture format. I think the last version of PSP that had the support for it was version 6 or 7 from 1999.
However Ultimate Paint supports LBM and has a freeware version:
http://www.ultimatepaint.com/download.php
Download the "Ultimate Paint Standard 2.88 LE (Freeware Edition)" from that site.