Actually not at all: well,
definitely not in the UK.
Here we use the Copine scale (http://en.wikipedia.org/wiki/Copine_scale)
It's important to note the ONLY classification that images of clothed children can fall into is #1 (the least "worst" case). However to actually qualify the poses would have to be very sexually sxplicit. Child underwear models, for example, wouldnt normally get categorised there (obviously if there is more extreme indecent material they WOULD be appended as "relevant" images).
Your right the jury will be shown an image and if it has a child in it they will cry "OMG CP" but fortunately experts analyse all of the data first and ensure only real indecent images make it to the trial :)
I cant say 100% for the US but I believe things work in the same way.
EDIT; a colleague pointed me at this (http://en.wikipedia.org/wiki/Miller_Test). Apparently in the US you lot are completely unable to make use of a simpel classification scale (I jest, I jest) so things are less cut and dried. In which case I guess your point stands.
I prefer our way.