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Old Entry (Information Might Be Deprecated)
It regularly amazes me how little people know about the capabilities of the .jpeg .jpg format and how, as a result, their image libraries are easily four to five time as big as they need to be.
The method that I am going to show here is going to reduce the size of every picture in a directory to about 500k. This compression will reduce the quality of the picture, but not enough to make it noticeable to the naked eye.
First of all, go download the excellent ImageMagick software at:
http://www.imagemagick.org/ (dl: win unix)
Once the software is installed, go to the directory containing the files (it would be wise to back-up your pictures first) and type in the command line:
mogrify -resize "2048x2048>" -quality 75 *.jpg
What it will do is that it will take each picture individually, resize it to 2048x[...] pixels and change the quality setting. I chose 75 because it yields the greatest compression/quality loss ratio. Check it out for yourself, I am sure you can't tell the difference between the compressed picture and the original. This typically reduces the size of a 3,000 Kb picture to under 500Kb.
Here are some other cool things you can do with ImageMagick intalled:
Get the picture properties:
identify -verbose abc.jpg
Compare the diff between two pictures:
compare -metric MAE First. png Second.jpg diff.png
Burn a logo into a pic:
(more on composing: http://www.imagemagick.org/Usage/compose/)
(Use white background)
composite -gravity NorthWest -compose color-burn logo.png image.jpg out.jpg
Write text on all pictures:
(x,y , dx,dy) 0,0 = original size
mogrify -draw "image color-burn 4,4 0,0 'logo.png'" *.jpg
convert -draw "image color-burn 4,4 0,0 'logo.png'" *.jpg out.jpg