This warning icon is shown, if iPhoto has lost the connection between the photo thumbnails and original image files and can no longer access the photos.
You mentioned that you imported the photos to Picasa. Have you used Picasa to access, move, or modify the photos? Does Picasa still show the photos? If yes, post back and tell us exactly, how you are using Picasa and iPhoto together and what you have done, before the photos were no longer visible.
if you did not move or modify any photos but with iPhoto, the first thing to do would be to rebuild your iPhoto Library: http://support.apple.com/kb/HT2638?viewlocale=en_US&locale=en_US
or simply to restore it from your most recent backup.
Are you talking about Picasa the application or the Web Site?
Using Picasa on an iPhoto is not advised, and can cause problems.
And what version of iPhoto?