lucy24 - 10:39 pm on Apr 28, 2013 (gmt 0)
My bad. I zoomed in on that "resized in HTML" line in the initial post. But I don't think there was anything about users uploading files. There I'd agree that you can perfectly well place the burden on them-- or their computers-- to do the work. In fact it's the same principle: the person providing the file should provide it in the best form.
For most people in most situations, maintaining multiple files on a server is probably much less work than dynamically resizing a single file. One possible exception is if you've got a nice header that you want to fit neatly into the user's viewport.
On the purely mechanical question: I suspect your human ability to keep track of things would fall apart long before the server. And if you're going to accidentally delete or duplicate* a directory, let it be one containing 500 files rather than half a million :)
* A computer doesn't absent-mindedly double-click instead of single-clicking. But a human does.