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There are two basic types of compression.
Lossless - You can get the exact image back when opening it again. e.g. png or gif
Lossy - Data is discarded. e.g. jpeg
Lossy formats are more compressed but each time you edit them they get worse.
It is better to save a master copy in a lossless format and then save them as jpeg's when uploading to the internet or sending via email.

type comrees-
.gif GIF - Graphics Interchange Format - Compuserve y lossless1
.png Portable Network Graphics y lossless
.jpg JPEG-JFIF - Joint Photographic Experts Group y3 lossy
.tiff TIFF - Tag Image File Format n2 lossless
.psd Adobe Photoshop n lossless
.pct PICT - Macintosh picture format n lossless
.cr2 Canon Camera Raw n lossless
.raw Raw format from Photoshop n lossless
1. Gif is limited to 256 colors, so if an image contains less than 256 colors it produces lossless comression, otherwise colors will be lost.

2. Sometimes a lossless compression algorithm called LZW is used with tiff images, but it is not universally supported.

3. Most graphic programs allow you to save jpeg images with different quality levels. Higher quality results in larger file sizes. Photoshop has levels 1-12, others use 1-100.

Output form Cannon EOS Rebel
10 MP camera 3888x2592 pixels
format size MB
.CR2 (camera raw) 12.9
.jpg (camera) 3.2
.jpg (PS 12[max]) 5.9
.jpg (PS 6[med.] 1.0
.raw (PS elements) 30.2
.psd (Photoshop) 27.9
.tif (Photoshop) 30.3
.pct (Photoshop) 27.9
See Image Filetypes.

PS - PhotoShop - Number refers to quality level.

Information such as small icons and thumbnail images are stored with the image increasing the file size significantly. You need to set preferences in your graphic program to save without these. They are usually deleted when you ftp to a web site.

cropped picture 322x207
JPEG level 3 - 29 KB

JPEG level 12 - 74 KB

tiff image was 283 KB


JPEG level 6 - 37 KB

png - 168 KB

See also Examples of converting jpg to png or gif in Photoshop

PNG - 98 KB

JPEG level 2 - 61 KB

gif was 115 KB

.gif 20 KB (.png is also 20 KB)

JPEG level 3 - 25 KB

.bmp was 152K

Examples of converting jpg to png or gif in Photoshop
Google Search for graphic format information.

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last updated 11 June 2007