I've been skinning XHTML now for 4+ years and this has always been an anoying one.

There are two choices here;

The column approach - put your image and text inside seperate div's, float:left; and set thier widths (percentatage totaling 100% is best). Then apply margin to the containing tags for vertical spacing. Don't forget to clear:both; after your column floats. 
Oh yes do watch out for overflow errors with IE (grrrr!), especialy IE6 this breaks columns due to incorrect calculations of widths, both padding and margins get included in the percentage calculations. This is best avoided by using percentages for your margin and padding as well as your widths... remember it all must equal 100%.

The absolute approach - place a position:relative; on your containing floated div and then use position:absolute; (with top and/or bottom, left and/or right set... this is required). This will position precisely from the edges of the containing div.

boo hiss ... tables bad .
Absolute version *should* play nice with resizeable content (so long as you don't fix the width and hieght) as it pop's the tag out of the normal page flow. It can however introduce problems with other elements that use absolute positioned elements... menus or javascript popup code.
Why is it never as simple as it seems?