We've already touched on Flickr as a source of good pictures to use in projects and presentations - including pictures from historical archives and museums. In this post, I'd like to cover two final points: finding images and downloading them for use.
There are a number of options available for searching through the billions of pictures on Flickr:
A lot of grammar school teachers use FlickrStorm because it allows the searcher to build a collection of images and then download that group of pictures in one fell swoop. I don't like this because I find people are not as diligent in keeping track of the sources of their pictures.

My personal favorite has the worst name of the group: CompFight. Using the settings above, you are sure to get a Creative Commons licensed photograph of high quality, without dubious content.
Once you have found an image that you like, copy the photo's title and artist, necessary for attribution:
The next step is downloading the image. Above the picture, click on the link that says "All sizes" This is only present if you searched (on CompFight) for "originals only" - only photos uploaded by a "Pro" account will allow download of high-resolution images.

I recommend downloading the highest resolution image available if you are placing the image in a Keynote or PowerPoint presentation or video. If you are using the image in a website (or a blog post), you are better off downloading a smaller image.
In terms of citing the image, strict adherence to the MLA format would require:
Author's Name. Title of work. Date taken. Online image. Image site.
Date seen. url.

Citing the above image would look like this:
ecastro. "That's my stapler". 16 September 2008. Online image. Flickr. 16
September 2008. <http://flickr.com/photos/ecastro/2868168916/>