Google's picture-sharing service, Picasa, has found favor not only with image-happy users, but also as a filter-evading route for spammers to stuff your mailbox with junk, according to a new Message Labs report.The latest bypass around your spam filters? Google's Picasa Web Albums, according to August findings from security company MessageLabs.

The company's August Intelligence Report identified the appeal Picasa offers spammers: filters tend not to block files from Picasa and similar services precisely because the services are so popular. Filtering would catch the user's legitimate image-shares as well as the spam.

Image-based spam has the additional advantage (for the spammer) of being essentially invisible to text-based filters.

The Picasa findings are a reminder of one of the dark sides of the cloud: hosted services can make effective, if unwanted, hosts for spammers.

MessageLabs noted that overall spam volume increased by 160 percent in August, much of it traceable to the spam surge created by botnets Srizbi and Cutwail, each of which increased mail volumes by at least 20 percent.

One other big trend noted in the report: a more than 50 percent increase in the number of spams posing as e-greeting cards or postcards.

Here's a good bMighty look at how to beat botnets.