The main image editor most people will recommend is GIMP, but depending on what type of image editing you're doing, Krita might be better. They're both available for Windows, so you can install them now and try them before you switch :)
GIMP is more like Photoshop, and can be made to look more like it with the PhotoGimp extention. Krita is more for digital drawing, but can be used for some photo editing too :)