Google Website Optimizer lets you test different versions of a landing page for effectiveness in achieving some pre-defined conversion goal.

https://www.google.com/analytics/siteopt/splash?hl=en

Note: If no-one finds your page via Google, this won't be much use to you!

Note2: You may have different results with traffic coming from other sources - eg. if most of your Google traffic is through searches for "giant spanners", GWO may suggest you get most conversions using the version of your page which includes a big spanner picture. However, the majority of your visitors, who come via adverts on "Claw-hammer forum" and are looking for your outsized claw hammers, may be put off by the spanner theme.