can someone better versed in Joomla than myself explain the phenomenon?

I installed a Joomla yesterday, and spent 4 horus this morning uploading tweaking images content for complete site.

then i uploaded 1 edited image and clicked preview and ALL the sample data was refreshed overwritten all the stories and articles i had put up.

what the f? any good advice? i have no explanation why this happened but don;t feel like making the same website twice.