JPEG image files are very small in size compared to some other image file formats like PNG or BMP because JPEG saves the image data in the compressed form. Suppose you have to take only a portion of a JPEG image (crop a region of the original image) and save it again as a new JPEG file, then imagine what will happen to the image data. The already compressed image data will be forcibly compressed all over again and this will result in the loss of the original image quality.
JPEGcrop is a free Windows application that can help you crop the JPEG images without any loss in the picture quality. Not only that, it can also perform some other lossless picture transformations like rotating the picture, flip the picture horizontally or vertically, turning the image grey scale and so on.
After opening the JPEG file in the JPEGcrop program, you can find all the actions from the toolbar. Typically, you have to select the region that you want using your mouse pointer. After this you can click on the Crop/Wipe Mask button to crop it and then save the image on your hard disk. Using the same steps, you can also wipe the selected region instead of cropping it if you have toggled the Crop/Wipe option in the toolbar to indicate that you want to wipe the region.
The way it works is by first decompressing the JPEG data, performing all the transformations that you ask for, and the re-compressing the JPEG file. This way the original quality of the picture is retained even after editing the image a number of times.
Conclusion: JPEGcrop is able to perform lossless actions on your JPEG files like rotation, cropping and desaturation. If you do not want to compromise on the JPEG picture quality then this program is for you.
You can download JPEGcrop from http://jpegclub.org/.