Hoople - 8:16 pm on Jan 20, 2013 (gmt 0)
brings up several sites with articles worthy of a good read. I myself would start with 'CSS Floats 101' at alistapart DOT com.
understanding the concept of floats and clears
Other browsers, particularly IE6 need 'special' source order treatment to display an identical layout. Changing the browsers window size will sometimes result in things moving in ways that 'break' the relationship between content and images. As lucy illustrated divs are a method to force behaviour.
BTW - on some discussion boards mentioning that w3 place may get a variety of negative reactions. Ditto for job interviews.