As a child I already loved flicking through magazines, just to look at pictures. The Internet makes it so much easier to absorb vast quantities of visuals. Simple add a huge amount of photography-focussed RSS feeds to your Google Reader and hold down your finger on arrow-down, only to lift it when an image instantly touches you.
Not that I am hunting for images to use in presentations, often they are not right, often they have copy right restrictions. But still, immersing yourself in images improves your slide design skills. It is a bit like the best way to learn a language: surrounding yourself with it (maybe Hebrew is the exception to the rule though).
The most interesting images are often not the most professional ones. Stunning sun sets, volcano eruptions, can be beautiful but do not touch you on an emotional level.
I recently added the RSS feed for ffffound to my Google Reader: a community image book marking site with a large readership. Images are picked by random users (most of them with a good eye for photography), as a result you get a frequently updated image stream full of surprises. As an example, here is an image I found today on the site:
Yay!Everyday is another example of image highlighting site, worth following.