Note that if you want to use the PTA to generate GAME code and custom piece set for a preset, you have the opportunity to still alter the suggested code after pasting it into the preset. In particular, when you want to use piece images that were not available in the PTA's table, you can pick some other image for them, and then change the filename of those in the custom set to that of the images you do want. You are not even obliged to use the custom set that is suggested by the PTA; it is just an optional service. If a piece set exists that has all pieces you need, you can simply use that in the preset, and you don't have to copy anything to the custom-set area. But you must make sure in that case that the pieces you did use in the PTA for generating the GAME code have the same ID as those in the selected piece set. This should not be a problem, as the PTA allows you to alter ID as well as move of the pieces.
Note that if you want to use the PTA to generate GAME code and custom piece set for a preset, you have the opportunity to still alter the suggested code after pasting it into the preset. In particular, when you want to use piece images that were not available in the PTA's table, you can pick some other image for them, and then change the filename of those in the custom set to that of the images you do want. You are not even obliged to use the custom set that is suggested by the PTA; it is just an optional service. If a piece set exists that has all pieces you need, you can simply use that in the preset, and you don't have to copy anything to the custom-set area. But you must make sure in that case that the pieces you did use in the PTA for generating the GAME code have the same ID as those in the selected piece set. This should not be a problem, as the PTA allows you to alter ID as well as move of the pieces.