Agh, I described "Trimming non-transparent areas" wrong. It works just as you describe - "trimimage (0)" will trim all horizontal and vertical borders which are transparent (or even a solid colour), leaving just the centre part of the image with all the detail. The zero controls the tolerance, so higher values should work even on JPGs with compression artifacts.
The script can be executed online, but yes, I run it on my laptop using a nice local webhost package called XAMPP.
Because ImageMagick is a commandline app, it can be used easily with many scripting languages, so perhaps Python or something similar might be easier than setting up a localhost just to use PHP.
EDIT: There is an even easier way, and it was right under my nose: ImageMagick includes a commandline tool called "mogrify", which is suited for batch processing. Best of all, it's cross-platform!
mogrify -trim -fuzz 0 +repage -thumbnail 256x256 *.png
This will do the same trim and resize operations on a directory of files as the PHP script above, BUT it will overwrite the original files.
Edited by Gnac, 01 November 2012 - 02:37 PM.