It could definitely be the number of images, if they're all being loaded onto the page at once (i.e. a page of say, 100 images), especially if they are large file sizes. Fetching images is a huge resource hog for the browser for any website. But, slow site speed could be a caching issue, plugin conflict, etc.
Before going to (expensive CDN), you should try the following things:
- W3 Total Cache plugin
- WP Smush.it plugin - optimizes images at import
- The #1 thing you can and should be doing: uploading compressed versions of the images you're sharing - you should be compressing images on your computer before loading them to WordPress. Even though WordPress may display a smaller version of the photo, if you're not performing this step, WP is probably loading a huge file size for each image.
CDN is more about maximizing site uptime, and assuring that if one server goes down, your site is still being served elsewhere on a regionally based server. Employing a CDN is expensive. You should try the free optimization techniques I listed above first before considering such dramatic action.