If while working on a web page or a document for print you need to compress a digital photo or image file to make it smaller, it is a good practice to save the photographs or graphics in their original uncompressed file format. Likewise, it is a good practice to create new compressed files every time a photograph or image is changed significantly (such as resizing or changing the file format).
This recommendation is the result of the compression technique used for JPG files (the most common file format for photographs and images). There are two data compression techniques: lossy and lossless. The lossless technique makes the file smaller by squeezing out inefficiencies in data storage so does not permanently delete any information from the file when the file is compressed.
Accordingly, all original information can be completely restored when a lossless file is decompressed. Lossless compression works best when compressing images with large fields of homogeneous color.
In contrast, the lossy compression technique permanently eliminates what it determines to be redundant or unnecessary information in the file during compression. While this can significantly reduce file size, it does mean that upon decompression, the file will contain much less information than prior to compression. With the lossy technique, every time a file is decompressed then recompressed, greater and greater amounts of information is lost and eventually there are too few pixels to properly render the image.
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