Probably because they compressed their images and others do not. Compressing images DOES degrade quality, but it is only noticable the more you compress. If you compress just a little, it can take file size down A LOT while still retaining most of the image's quailty. Also an image's resolution doesn't necessarily have to do with load time. I can have an 800x600 image that is smaller in file size and loads faster than an image that is 640x480. It's all about compression and HOW the file was saved. GIFs normally tend to load faster than JPGs because GIFs only support a smaller amount of colors. and JPGs tend to load faster than PNGs because PNGs support more colors. Of course, you can have a JPG load faster than a GIF and a PNG load faster than all three, it's all about compression I can put up examples if you'd like!

Karen is 100% correct about everything else though