I have a WordPress site which is 90% based on images.
I've installed Wp-Super-Cache, now it seems to load a little bit faster.
I've made a speed test on Pingdom Tools:
And also the Time Spent per Content Type:
So the problem is obviously the images.
My plan is to try as much as I can to optimize the images in terms of size, and then to add CDN support.
Is this enough in terms of improving the loading of images?
Serve your images through a Content Delivery Network. Amazon has Cloudfront which offers pretty cheap services and allows for Origin Pull which means the images are hosted on your server for origin storage, but cached and delivered from the CDN's nearest endpoints to the customer. Basically you set your images to be delivered through a CNAME on your domain like media.example.com that points to Amazon's assigned image server FQDN.
Your server then can concentrate on content instead of being mostly an image server. The only reason for optimization of image service after this is when content TTLs expire and the CDN has to reload. We cut our server traffic to a quarter of what we were struggling with when we implemented the Origin Pull system.
I just found out yesterday Google has a PageSpeed add-on for browsers that I'll suggest using as it breaks down performance issues by prioritizing importance. It also works in the browser (Chrome or Firefox) as an add-on. It also explains problems and solutions more if you're unsure what each item really means.
I ran PageSpeed on your site and my suspicions on the images being unoptimized were confirmed and it was the number one thing listed. Once fully optimized, it says you can reduce your image sizes by 43% (over 1.5MB). You can learn more about it on best practices page that talks about optimizing images.
Some other major things you can do are Gzipping your content (don't gzip images, though). This can save you half a MB almost. You may encounter problems with the PageSpeed website confirming your files are gzipped, but the website I linked and the PageSpeed add-on may work better for you.
It also says your images are too large for the size you're using them. This can save you ~.25MB.
It also suggests minifying your javascript and css files.
There's more suggestions by PageSpeed but these should lower your page size significantly.
I'd also recommend double checking how your caching plugin works as PageSpeed also suggests to add expiration headers to some files (it's easy if you have access to your .htaccess file).
Overall, get PageSpeed or YSlow for your browser. They'll tell you similar things (I just have a preference for the former as it works in Chrome's developer tools panel.)
I suggest some kind of progressive loading system, so that images farther down the page won't load as soon as the page does.
