This is an interesting problem. One solution (simplified) is:

$canvas{width} = 8.5;
$canvas{height} = 11;
foreach(@pictures_to_canvas){
    if ( ($canvas{width} > $$_{width}) && ($canvas{height} > $$_{width
+}) {
        $canvas{width} -= $$_{width};
        $canvas{height} -= $$_{height};
    }
#Throw exception here if height or width goes below a min value.
}
...
[download]

I hope this code makes sense. If you make width and height "properties" of the photos, it's just a matter of dividing the canvas into vertical and horizontal components, iterating through the photos, and throwing an exception when you've gotten too much on the canvas (or at least, that's how I've thought about it).

This would even work with while/until/etc... loops, but I thought the foreach was the simplest.

Hope this helps just a tiny bit.
Theodore Charles III
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perl -e "map{print++$_}split//,Mdbnr;"