spudje, are you using a tool named "FullPalette" to lock wb colors? If you don't, the colors for backdrop image are taken from free pens, but not all colors needed are available and WBPattern doesn't lock them all nor caring about precision. As a result, all images get degraded.
Using FullPalette allows to lock colors (say 64 of your 128 available). Loading a fine dithered 64 color image with its 64 colors locked and loaded gives accurate results. Problem is to get those 64 colors loaded because FullPalette don't care for pen position when palette entries are imported.
You need to fix this manually each time, if images are not rendered with a generic palette.