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to achieve this you'd have to modify some code, and its not easy (many things to consider)

NOT recomended at all

why you want this?¿ you can have it the way you want it using the css properties (the way it is now), and you mantain the original file with all its quality...

from experience i know a lot of people just simply upload a scan or photo and these can be quite huge. ofcourse they could resize before upload but thats as strange as it might sound not something people like to or can do.

So the easiest way would be to resize to a max (definable) size when uploaded.

well, in this case you'll have to treat the images as thumbs (that are automaticly created on the proper size every time that you upload an image)

you'll have to add some PHP code, so you need some skills or at least some knowledge on PHP.

if you know PHP, I'll tell you what files do you have to modify

That would be great I was thinking about that as an option too.

I have no problem understanding php, though probably not as well as you do