Oracle Forms

Hello, friends!
I am working in Oracle Forms 6.0.8 (that ancient tool) and encounter a problem with displaying TIFF images.
I have a number of scanned images and some of them are displayed and some are not. I've tried both extracting them from the database or from the file system (using READ_IMAGE_FILE). Sometimes I am getting just blank field in my image item and sometimes (after using READ_IMAGE_FILE) I get the error FRM-47100 (cannot read image file). Of course I've checked that all the files are displayed using standard tools for displaying TIFFs.
Basically the question is what kind of TIFF is considered valid and readable by Oracle Forms.
Analyzing TIFF tags actually gave me nothing: I have a pair of files with tags different only in image width/length and a number of rows/strip (but even those values are close), and one of the file is displayed correctly while the other is not. The thing I noticed is that invalid files are using compression type of CCITT Group 4.
Possibly Oracle Forms follows TIFF specification only to certain extent and does not support all the extensions, while sometimes the problems are not visible to user.
As a result of my work I need some automatic tool that converts undisplayable TIFF files to displayable ones but firstly I am to determine where the problem is. And it would be very good to have a prooflink that approves my decision.

Looking forward to any help! Thanks in advance.