There are in fact 3 overlays: Small, Medium and Large. In DatEdit, you can choose from these 3 only, because it wouldn't make ant sense to do otherwise, but in Arsenal3 thisimages.dat property was just a number - a pointer to images.tbl.
Launch up Arse3 and look for the Shield OL property. Write down the values that appear in it, then launch up TBLPad and go to the appropriate lines in it. Voila - you have the files you wanted