Hard to tell from your description but if you are getting a message about a plugin with a name ending with .png then you are infected with with one of the flashback trojans. Install the most recent java update which advertises itself as being able to remove the trojans.
If you don't want to do these installs then at least go to F_Secure's Flashback Removal Tool web page, download their Flashback trojan detection/removal tool, and follow the instructions you find there.
Your Mac is probably infected with the flashback trojan but it's relatively easy to remove.
Install this update > Java for Mac OS X 10.6 Update 8
That will remove the variants of the flashback trojan (malware).
Restart your Mac after the update is installed.
If that didn't help, use this removal tool > Flashback Removal Tool - F-Secure Weblog : News from the Lab