There is no difference in scanning roms or pngs. Generally you have to follow the storing methods:
rompath\setname\file 1...file n for decompressed sets rompath\setname.zip (.rar/.7z) with file 1 ... file n in the archive for compressed sets
So if the flyers datfile got 1 set named "flyers" with >1000 pngs in it, you need to setup your rompath either to the folder where the flyers.zip is stored (unlikely you're using compressed flyers since it's > 4GB) or you set your rompath to the parent folder of the folder flyers.