Free Photos for Presentations?

I do quite a bit presentations. To make things more interesting, usually I put interesting image or photos in my slides. (I know not to over do with the images and such.) The question is

where can I get good and free photos for my presentations?

I see those presentations at slideshare.net and envy those great images. Aaa...

what a mess (cabling and stuff)

taken an hour ago at home ...



that's not all. the rest? even worse ... ha ha ha