For anyone having issues building larger apng files with the image to spinball app, replace the apngasm.exe file in the apps Resources folder with this version;
apngasm64.zip
(Note - this is probably only needed if you are using a 4k underlay image and with a lot of animation frames, which in turn uses a lot of memory)
Thanks @Mike_da_Spike
I'll update the copy in the main download in the next app update