I don't know for which reason, but usually jpg images give bad artifacts on Kindle (PW at least). For minimize this issue, please use GIMP to export them, with the settings shown in GIMP_EXPORT_SETTINGS.jpg.
Before exporting, you should also desaturate the image, being the Kindle screen a gray-scale only type. In order to do this, in GIMP just click on Colours->Desaturate and choose which kind of desaturation looks better for your specific image.
Don't forget also to scale any image to 150x150.

Thank you!

Aeris
