Yeah it took forever to figure out, especially since I have basically zero JS/HTML knowledge. Happy to share though:
Make an unlinked page and make it your home page.Add a code block and paste this:
var myimages=new Array()
//specify random images below. You can have as many as you wish
document.write('<img src="'+myimages[ry]+'" border=0>')
Make sure that for every image you add you update the number next to myimages[x]. The whatever.com URLs are the direct URLs of whatever image you're using. All the URLs I used were from pictures I'd already uploaded on other parts of my squarespace site.