Why resize the images at all?  Telling the browser to resize an image with html accomplishes nothing because the file size is still the same.  I'm on dial-up and a half meg image at 1024x768 takes just as long to load as a half meg image at 640x480.  Resizing via html doesn't increase the speed...it just butchers the image and makes it look horrible.  Now, if the software actually resized and resampled the image and reduced the file size, that might be a different story and useful to some.  But, all that's happening is people are getting irritated looking at bad images and being forced to click once again just to see a piece the way the creator intended for it to be displayed.

HTML resizing = bad idea

If you insist in keeping this "feature", I suggest giving people the option to turn it off.