Perform a lossless transformation on the image in a JPG file. This can be used to perform one of a set of simple transformations without decompressing the JPG data. By not decompressing, you don't have to re-compress, thus avoiding adding additional JPG artifacts to the output image.
This works by rearranging the compressed data, without ever fully decoding the image. Therefore, its transformations are lossless: there is no image degradation at all, which is not the case if you were to use any of the other image operations to a JPEG.
The transpose transformation has no restrictions regarding image dimensions. The other transformations operate rather oddly if the image dimensions are not a multiple of the MCU size (usually 8 or 16 pixels, though sometimes 32 or 48), because they can only transform complete blocks of DCT coefficient data in the desired way.
The default behavior is to discard these "extra" blocks. This prevents ugly strips of unmodified data along one edge. You can override this discarding of extra blocks by selecting Do not trim extra MCU blocks, though it is not recommended.
Another option is to convert the JPEG to grayscale by checking Transform to Grayscale
This Image operation will only work on JPEG's and is ignored otherwise. In addition, any other image operation will be ignored if the image is a JPEG.
Possible Transformations are as follows:
See examples & screenshots on the web.