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For your first manipulation you did it very well! but here are some things to think about next time <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile)" />

Look at the lighting, you did find two similar subjects, but I would suggest that if you didnt' already, use dodge/burn (LOW OPACITY) to get the right lighting.

Blur...Notice that the BG was probably a smaller size than that of the equid, so I would have sharpened it up a bit, then blurred the edges to make itseem more real.

Horse Outline is too harsh, when you position the horse the exact spot that you want to, take the blur tool set it at 100% and a small brush (about 20 px for the size of the photos) and blur the entire outlines of the horse, that way it flows between photos.

Coloration. Notice that there is a TON of green and yellow tones all around the horse, but none on it. (I have a hard time with this too!) use the color adj. selection, OR make a new layer over the horse, and set it to overlay, pick darker greens and yellows and put them in random spots of the horse (Legs, muzzle, hair, around the eye, etc) and use a pen pressure brush to lightly put it on the horse..

other than that, you did an awesome job, and should be proud of what you accomplished~! <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile)" />