JPEG files typically have a .jpg file extension. If you can't see file extensions, you may want to enable windows file extensions[*] as you will have great difficulty editing and manipulating image files otherwise.
Digital cameras typically default to saving photographs in JPEG format as it provides a good compromise between quality and filesize for most uses. JPEG is also a good format for delivering photographic images over the web or in email.
JPEG is a lossy compressed file format, this means that a lot of information that is imperceptable to the human eye is discarded - Converting to another file format will not restore this missing data.
Unfortunately this degradation can become perceptable if an image is later manipulated, so if you intend to produce high-quality work you might want to switch your camera to take photos in lossless TIFF, RAW or DNG[*] format instead.
There are a small number of JPEG manipulations that can be performed losslessly, such as cropping, rotation (at 90 degree angles), flipping and removal of CCD artefacts such as bad pixels (see Jpegpixi).
Modern cameras save a lot of information about a photograph (time, lens and exposure data) within the JPEG file as EXIF data. If you are having trouble, one of these causes may be to blame:
- Rotating a photo may simply flip an EXIF tag from Landscape to Portrait without changing the image itself. The resulting image may or may not appear rotated depending on the viewing software.
- Rotating a photo may rotate the image without changing any EXIF data to suit, this may cause problems when a GUI tool reads the Field of View from the file. To avoid this, use a tool like photomolo[*] to do lossless rotation and let it reset the EXIF tags appropriately.
- Opening and saving a photo may discard all the EXIF data, switch to another image editor.
Color downsampling issues
JPEG isn't an RGB file format where each of three red, green and blue channels are saved alongside each other. A JPEG image is typically a monochrome image with two lower resolution color channels.
This means that applications where color integrity is important like correcting chromatic aberration are not suitable uses for JPEG data.
Other JPEG limitations
JPEG doesn't support multiple layers, alpha channels or high dynamic range which means that it isn't much use for work in progress either. When working with such data, lossless file formats are preferable such as PNG, TIFF, PSD or XCF[*].