Just to make it clear, there are three versions of the bitmap/keyboard files. The errors you are getting may have something to do with which one you are using. Please use the proper set for your screen size.
QVGA
Download:
VGA (Shaded buttons)
Download:
VGA (Plain buttons)
Download:
Here is the documentation for the calculator:
HP 48G Series Quick Start Guide:
Detailed information for HP 48G Series Quick Start Guide
HP 48G Series User's Guide:
Detailed information for HP 48G Series User's Guide
HP 48G Series Advanced User's Reference Manual:
Detailed information for HP 48G Series Advanced User's Reference Manual
Note: The Advanced User's reference was NOT provided with the calculator. You would have to buy it separate. Now you can download it for free.