as jason says Image size is important.
Remember the average viewer has an attention span of 15 seconds hence why we use thumbnails.
So there is no point is using Hi def 13mb images, better to use photoshop etc to batch reduce the images to the maximum size required for your site.
Standard resolution for designers now is 1280 * 1024 but your image area will be around 500 * 400 so I'd convert all images to 1024*768 in jpg which will reduce size radically.
It will reduce your memory overhead and save you oodles of time.
Also with all due respect to WA and others whilst resizing using PHP etc is ok it is not as good as using a decent photo editor with all the algorithms. So reducing the image size before uploading may improve the quality of your end images