tl;dr if the PNGs are not transparent then you can't overlay / composite!

Pontifications

convert 0000001-first360-flickr-roland-2004-12-avgcolour.png 0000002-first360-flickr-roland-2004-12-avgcolour.png\ 
-composite 1-2.png
convert 1-2.png 0000003-first360-flickr-roland-2004-12-avgcolour.png\ 
-composite 1-2-3.png
convert 1-2-3.png 0000004-first360-flickr-roland-2004-12-avgcolour.png\ 
-composite 1-2-3-4.png
convert 1-2-3-4.png 0000005-first360-flickr-roland-2004-12-avgcolour.png\ 
-composite 1-2-3-4-5.png

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