Hi James, can you send the contents of the log file that PixelCNC produces after an image-loading failure? You can get to the log files folder from the File menu. I'd suggest running PixelCNC, opening the log files folder, and then re-creating the crash by creating a new raster layer with one of your larger images. Then if you sort the log files by "date modified" you can be sure that the most recent one captured a session where the issue occurred. You should be able to open up the log file and copy-paste its contents over to http://www.pastebin.com which will generate a link you can post here so that I can take a look at what PixelCNC is doing. :)
Charlie