read the rest of his post
making selections is too much to type
there are many techniques and many tutorials
basically once you got the wheel selected just open car image and drag the
selection over the other picture (or copy/paste)
then use edit>transform to resize or change perspective to fit
if the wheel is the same for front and back
drag wheel layer to the paper icon to dupe it and use move tool to place
over other wheel
save the layered doc as PSD in case you need to edit
and use save as.. or save for web to make a flattened copy