It's not perfect, but it works:
-) First you scale the image twice big as you want to have it in the final result (in that case 200 px)
-) Then you resize it to 50% -) Then you crop it
The last "strip" strips off exif infos from the image.
I've never used ImageMagick, and so I don't really understand the filters you describe (Is there somewhere an explanation about how to use these settings ?), I don't understand what does mean %1,...
Is it possible to do such a thing with standard GD2 ? In PHP manual coding, what I usually do is first measuring the image, take the smallest value and crop with that value, and then I resize. The thing is that here I can't know the size of the image by advance... So I can't crop precisely.
Is there an issue for that ?
And about ImageMagick, do I have to change something in the image.ini.append.php to use it ? I think I must have ImageMagick installed on my computer, ain't I ?