Optimum photo size for quality & speed

I was out and about today and for the first time NOT connected to fast Internet, but using my Samsung Phone on a 4G network and the pictures took too long to load. The site is http://sharkshack.org and the photos are in Menu Items. There are quite a few and many are .7Meg to .9Meg in size. Of course the idea was that the slider photos be appetizing and vibrant... that they are... but takes too long to load them all. So do I: Preload Images? or Shrink their size? Comments and or suggestions welcome