how to switch driver control from the top window to iframe#mainFrame

I am using firebug to inspect the xpath, one block of firebug shows the element your finding is in window or iframe and so on. I am facing similar kind of problem where it shows me 2 options 1: Top window 2: iframe#mainframe now the problem is,when i inspect the element it shows in iframe#mainFrame, i tried switching the driver control from main window to iframe and detect the element using webdriver but it didn't work for me,I wrote the following set of code:

driver.switchTo().frame("mainFrame");
driver.findElement(By.xpath("//div[@class='div_img']/img[@src='http://ser/themes/20/3/Flor/CF.jpg']")).click();

Note: when i checked the iframe in the html code,it doesn't contain another document inside it, It only contains id and other style and all and src tag.
Kindly suggest if there is some other way through which i can detect the element.

via Chebli Mohamed