I've always argued/thought that deconstructing an image that really moves you is a vital and perfectly valid creative tool.  I spent almost the whole of my 2nd year as a Fine Art student painting a meticulous full-sized copy of "Mariana in the Moated Grange" by Millais - my lecturers were nonplussed, but I learned an awful lot about Pre-Raphaelite painting techniques!  Can't see a problem with the ethical side of it, as long as you don't try and pass it off as your own work or make money from it.  As others have said, it's always good manners to ask, if in doubt.  I usually find it opens up other creative avenues of your own as you go along, anyway, sometimes far removed from the piece that inspired you in the first place.  I'm always trying to emulate Rochr in Bryce.  And failing!

My self-build system - Vista 64 on a Kingston 240GB SSD, Asus P5Q Pro MB, Quad 6600 CPU, 8 Gb Geil Black Dragon Ram, CoolerMaster HAF932 full tower chassis, EVGA Geforce GTX 750Ti Superclocked 2 Gb, Coolermaster V8 CPU aircooler, Enermax 600W Modular PSU, 240Gb SSD, 2Tb HDD storage, 28" LCD monitor, and more red LEDs than a grown man really needs.....I built it in 2008 and can't afford a new one, yet.....!

My Software - Poser Pro 2012, Photoshop, Bryce 6 and Borderlands......"Catch a  r--i---d-----e-----!"