To answer some questions:
– Brushes went from 500 to 999 pixels with version X. Then it went up to 3000 pixels at version X8. So for brushes, it might be important to split those small ones that can be used in any version and the large ones that can only be used in X8 and above.
– 8bf files are filters files, however, some are quite old and might or might not work with PSP. Some of them will work only with the 32-bits version of PSP or only with older versions of PSP (as they were all 32-bits back then). Some are just no longer working with newer versions of PSP because they were never updated for the new “engines” so to speak. Try them with the oldest version of PSP you have.
