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Here’s another awesome slide deck from the Velocity conference - Image Optimisation, 7 mistakes and how to correct them (PPT) by Stoyan Stefanov.

In his audit he has found 45.6% of the total page download size of the top 10 websites homepages are images. Here is the list of 7 tips for improving image optimisation from the slides.

1. Use PNG’s over GIF’s - The big sites (Yahoo / Google /Blogger / MySpace / Live.com etc) could save 20% image size by using PNG’s over GIF’s.
2. Not crushing PNG’s - Default PNG setting are not the most optimal. Using pngcrush can reduce the size of PNG’s even more.
3. Not stripping JPEG meta data - Jpeg contain lots of meta data including in some cases thumbnails of the image and audio inside the EXIF information. (Remember thumbnails should be 3-4K in size)  Strip out all non critcial exif information using tools like jpegtrans
4. Use Palette PNG’s instead of True colour PNGs.
5. Avoid AlphaImageLoader, try PNG8 or at least _filter
6. Crush generated images - GD and Imagemagick don’t always do the best job in optmising images.
7. Use CSS sprites!

Finally Stoyan says if you optimise your images you can save 10-30% of the download size.

This entry was posted on Wednesday, July 2nd, 2008 at 12:36 pm and is filed under Speed Up NZ Internet. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.