Large image sizes slow down a website more than anything else. Be sure to reduce the size of your images before uploading them to your site. If you have Photoshop, use it to save images for web, otherwise, here are some recommended free alternatives.
I recommend using TinyPNG to compress images before you upload them to your website.
If you already have a bunch of images on your site, use Imsanity. This plugin will resize previously uploaded images and will automatically resize newly added images.