High- and mid-range cameras often take pictures in the RAW format. This format is not compressed and hense are not saved as JPG images. The good part about this is that images can be compressed in a format which does not compromise quality. The bad part about this is that they require an image editor capable of dealing with RAW images.
RawTherapee is a free graphics editor for RAW images. It can open and edit photographs created by high-end cameras like Nikon, Sony, Canon, etc. The most common formats are NEF, DNG and others. If you don’t have a camera which produces RAW images, it can also open and edit JPEG or TIFF images.
The file browser lets you view images before you open them. The previews it generates are of high quality, it even includes a scoring system for classifying images.
RawTherapee provides a lot of high quality image editing features for free and even outperforms some of the expensive RAW photo editors available.
RawTherapee can apply special effects to a picture and convert RAW to JPG.