300 core-seconds per hour means that you have the equivalent of a single-core (x86-type) CPU operating at 1.8GHz for 300 seconds (5 minutes) each hour. If you try to operate a site/script that makes very large, computationally-expensive use of the server, you can completely use your allotment up with only 10 page views per hour. Thankfully, most web pages/scripts are not nearly that intensive. On the other hand, Joomla (and other content management systems, or CMSs) is "fat"; it does an awful lot more processing and database querying than a custom-designed site that does the same thing would, so if you have a high user base or a lot of dwell time (users spend a lot of time on your site) and short pages (users open many pages rather than reading longer pages for a longer time), you can run into the limits pretty quickly.
Images have nothing to do with CPU limits, and there is no limit (as such) on bandwidth. However, 100 HTTP requests for anything on a web page is way too much. If you're talking about one image that is displayed 100 times, that's not bad, but 100 different images is guaranteed to provide very poor performance.
Clicks have nothing to do with anything; what matters is what's happening on the server.