create thumbnail or use large HD image twice?

Question

I have a slideshow which will be displaying large 400px X 1000px images. I need to have thumbnails of each image in the slideshow, so I was wondering... will the page load faster if I simply use a second instance of the image but lower the width and height - will the page load the large image twice? Or would the page load faster if I load the HD image and a smaller thumbnail?

Answer 1

Generally, I would advise against using images that are larger than the intended use. I would, personally, definitely recommend serving a properly sized image. These recommendations are based less on page load time (as the difference in your case is probably negligible) and more about best practice.

A very handy php utility I've use a lot for stuff like this is timthumb

Answer 2

Generally it's a bad practice to create an image gallery of any kind using only full size images, lowering their dimensions when using as thumbnails. Yet if your case is a small gallery with < 30 images displayed per page, then this might do. In the other case scenario, imagine that a page has to preload all of the big images no matter if user would play the whole slideshow or not. This slows the page load time and generates unwanted data transfer. When it goes to timing, it would still be faster to generate thumbnails. A good tool for that could for instance be phpThumb which is able to generate any image size from its original and cache the output afterwards, speeding up the page load next time.