If your mouse does not scroll horizontally and/or you don't have a trackpad, try using the arrow keys on your keyboard to scroll through your slide tray.
If your slide tray or your image search results on the web are scrolling automatically / jumping to the beginning, the first thing we recommend is to try another browser. So far, this issue has only been reported by users with outdated versions of Safari. We recommend trying Google Chrome, but you can certainly check for software updates to see if there are any updates available for Safari.
If this still does not help, please click the "submit a request" link at the top of this page, and let us know what browser you're using (if you're using Safari, please click "Safari" in the menu bar at the top of your screen and choose "About," and let us know what version you're using - even if you've recently updated, since different versions are available for different versions of OS X) and if you're using a trackpad or a mouse.