Images & illustrations make content lively & sticky. Commercial websites can affoard to pay for slick images but thankfully for the rest of us, there are a good many sites that offer free photos & images.
I spotted a list of over a dozen such sites with liberal licensing policies on the morgueFile website (at the bottom of the home page). morgueFile allows usage without attribution.
Smashing Magazine also has a nice compilation of free stock photography sites.
Here are a few more obvious & popular ones that both the lists missed (sites are linked to their pages on licensing terms):