How to empty iPhoto

Not sure how, but I seem to have unknowingly imported more than 1000 pictures from lots of different sites. They include avatars, smilies, text from web pages (those are weird - like screen shots, but not my own), hundreds of pictures of different computers from various vendors. Don't have any idea why they're being saved.

Is there a way to simply clear iPhoto of all the pictures at once? I've tried to "Select All" with the intention of sending everything to the trash, but only one picture at a time goes away.

Since I don't know what I've done to cause all these shots to be saved, once cleared, how do I stop this from happening again? Geez, I feel like a dummy!

If you really want to completely obliterate it, just delete the iPhoto library from the Pictures folder in your User directory. To stop it happening again you'll have to figure out how it happened in the first place

Thank you, the pictures I wanted to delete are now gone.

If anyone has an idea on how I may have done this, please let me know; I can't figure this out. FWIW, all browsers are set for downloads to go to the desktop, and I don't see any settings within iPhoto which would allow this. Thanks.