Sometimes printer drivers won't use the black cartridge to print photos/graphics. Only for text. So for color photos it makes dark colors (including black) by mixing the other colors. And you could get gross looking pictures as a result
You might want to check the printer settings in paint.net for anything that might cause this kind of issue, like:
(1) Is the app doing color management? Maybe try to let the printer manage colors. Photoshop has this.
(2) Is there a setting in the printer driver to explicitly tell it to use the black cart on photos? Maybe enable it.
(3) Did the paper settings somehow get messed up? The amount of ink it puts down is tied to paper settings so if you get it wrong...
GIMP is okay when you absolutely have to use it. But it's still not great.