input: [Metric] Backup image [Description] 
output: The number of times that a backup image is served in place of a Rich Media ad. A backup image can be shown under these circumstances: The user is using an unsupported browser or operating system. The Flash application isn't installed on the user's browser. The Flash-published version is higher than what is installed on the user's browser. The publisher implementation of iframe is incorrect or invalid. Note: This event isn't available for non-Rich Media ads.