Oh dear, I was not aware that the GoG version used png instead, sorry about that. I feel bad that this tool doesn't support the GoG version correctly, because I love GoG's business model. Unfortunately, making a patch of this program for even a slight fix like this just isn't high on my priority list at all.
What's also unfortunate, is that Serri's solution won't work because I designed Plus Porter in a way such that all the program's features all locked until an installation of CS+ has been verified, and its way of doing that is searching for all the image files in the game's directory, which it expects to have the .bmp file extension. I made it so stringent like this because the admins and moderators were very serious at the time about copyright infringement with Cave Story+'s graphics, so I wanted to make sure I stayed in DT and Andwhy's good graces. I don't know if the GoG CS+ .exe will work the same with all those images if they're converted to .bmp, but you can try that.
Here's my suggestion:
First, install an image batch conversion/editing program. From a cursory Brave search,
XnConvert looks promising. Tell it to convert all of the png files in the game to bmp. If the game doesn't like that, then you can have it simply copy all of the png images and create bmp copies of them that exist alongside the existing png images. Then Plus Porter should work fine. The one extra step that will be required if you still need to keep the original png images, is that Plus Porter is going to make the necessary changes only to the .bmp files, so once the mod is applied, you're going to need to convert the newly edited bmp files into png equivalents to replace the originals. The edited images can be seen in the folder that the 4th Ending mod patch comes in. It should go without saying, make sure you have your game backed up or a means to re-install it if you mess something up in all of this image rearranging.
So sorry that you have to use such a hacky solution to get this to work!
