For Windows users only, IrfanView is a small, but very capable free image editor. It handles most types of image formats and is capable of batch conversions to automatically edit or manipulate a set of images.
There are also many plugins available to expand the feature list. When I am using a Windows machine and need to do basic image editing tasks, I prefer to use IrfanView. With the small file size, IrfanView opens amazingly fast. Typically, I can open IrfanView, crop my picture, resize the image, save, and close by the time Adobe Photoshop has opened.
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