Based on the information regarding file systems, FAT32 can't use files over 4 GB. If you're trying to transfer a file from PowerISO to a FAT32 and it's bigger than 4GB, then an error will appear. You will need NTFS for this procedure.
Secondly, PowerISO support disc image files with FAT12, FAT, FAT32, NTFS and ext2, ext3 partitions. In the end, this is rather a problem with the files you're operating with, because, as you can see, PowerISO supports these file systems.