F

Program Files\Sports Interactive\Football Manager 2006\data\graphics\pictures\kits\clubs\eng (or the applicable league).
Then, grab a config.xml file from any old kit and you'll have to change the lines that look like this:
<record from="everton_home" to="graphics/pictures/team/650/kits/home"/>
to:
<record from="[image name]" to="graphics/pictures/team/[team id]/kits/[home/away]"/>
...or rather, add them on at the end. Someone else may come along to explain things better - sorry if this is a bit garbled.