How Reduce Picture File Size ?

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End of November 2009 Google informed they are adding one more factor into their ranking algorithm - that is webpage download speed. As this is only one of many factors it will mean that a fast downloading webpage should appear at the top of search engine results while a slow webpage would appear closer to the bottom / end of the results pages ( for the same search query ).

It's therefore very important to have fast downloading webpages. Images are usually what makes webpages download slowly so one needs to make picture file sizes as small as possible.

Main Factors Determining Image File Size

Big or small physical size of your picture (cm x cm, in x in, pixels x pixels).
Great or small number of colours.
Not-interlaced or interlaced GIF format. (background pictures always "normal")
Non-progressive or progressive JPG format. Progressive JPG gives a slightly bigger file size BUT reduces display time. (background pictures always "normal")
Non-packed or packed GIF or JPG (= "Save for the web").

How to Reduce Picture File Size

When you include pictures on your webpages the picture file sizes will determine the download time for the webpage. You can always put small pictures on your webpage and thus keep download time at minimum. If you offer the original good quality picture in full size, you can further reduce the quality of your small pictures. In a small size picture you cannot see many details anyway.

Reduce physical size, you can make a separate smaller version for your webpage. Often a picture width of 200-150 (or even less) pixels is quite enough for a webpage. Smaller physical size means less pixels, which means smaller file size. (Both .jpg and .gif format)

Cut anything unnecessary out of the picture (both .gif and .jpg) - a further reduction of physical size. See How to Reduce a Picture Size.
Reduce contrast. Keep both original and preview version in sight. Reduce the contrast as long as you don't notice any difference in the preview. Less contrast means less picture code, which means smaller file size. (only .jpg format)
Reduce saturation. Less colour definitions gives also less picture code, which means further reduced file size. Sometimes reducing saturation gives a slightly darker picture. If so, increase brightness just a little - the increase in file size of this is very small. (only .jpg format). Saturated colours are intense, pure colours without any gray. When you "de-saturate" you make the colours more dull but human eyes are not very sensitive to this.
Reduce number of colours. This you do only for .gif format images. For most .gif images you can reduce number of colours to 16. Don't go below that (except for black and white ones = 2 colours), the result can be unpredictable in some browser versions.
Increase compression. This means in fact you reduce quality - often you can reduce quality by 50%, i.e. you use 50% compression instead of 10% or 20%.
Digital photos. When you get a photo taken with a digital camera in .jpg format it can be difficult to reduce file size. What you do is you change to .png format first, then proceed as suggested above.

How to Reduce Picture File Size ?

Main Factors Determining Image File Size

How to Reduce Picture File Size

What is EXIF data ?