Are you limited to Selenium only? In case there is an option for you to try something else for this task, you could do that with a visual regression testing tool. Such as Screenster (http://screenster.io/) which is actually an alternative to Selenium for visual/CSS testing tasks.
Screenster is a test automation tool which performs screenshot-based comparison of different versions of your web pages. First it creates a visual baseline for a page, taking a screenshot for each user action. During the next run it takes a new screenshot at each step, compares it with the one from baseline and highlights differences.
Screenster is a test automation tool which performs screenshot-based comparison of different versions of your web pages. First it creates a visual baseline for a page, taking a screenshot for each user action. During the next run it takes a new screenshot at each step, compares it with the one from baseline and highlights differences.