Além do IrfanView (veja acima), você pode dar uma chance ao Paint.NET (faça o download, por exemplo, aqui): link
If all you want is a quick image retouch, Photoshop is overkill. Paint.NET, on the other hand, is an open-source freeware editor with all the essentials, including tools to crop, rotate, and resize images, adjust colors, and create collages. It supports common image formats--JPEG, PNG, GIF, TIFF, and others--but not high-resolution RAW files.