Thank you
The dog head was originally distorted, a pic of a dog sniffing the camera was the original for that Photoshop competition. That's where the idea for Trinity's gun came from, the head and the gun had the same close-up distorted perspective.
Matching the colours has a lot to do with firstly getting your levels and curves right (NEVER use auto anything) so the shadows match (use the sponge tool if the shadows are way off), and then using hue/sat and colour balance to get the tones of the colours right. You're just going to have to fiddle with the colours until it looks right. A good tip is to use Layer -> New Adjustment Layer everytime you normally would just use anything from the Image -> Adjustments menu, as this way you can go back and tweak everything or even disable something that isnt working really easily.
The melding from the gun into the dog head is done with a lot of masking. Masking (Layer -> Add Layer Mask) or hiding pixels is always better than outright removing them as if it doesnt look right you can go back and un-erase bits that otherwise would have been deleted. Using a small round brush and gradually scaling the flow down will give the best results.
Another technique I cant help but mention is to the gaussian blur outline. There's nothing better for getting rid of jagged edges and making something look real. Once the object (for eg. the dog head) is in place relatively well cropped and colour matched, with the dog head layer selected click the magic wand anywhere execpt on the head. You should have everything except the head selected. Click Layer -> Add Layer Mask -> Hide Selection. This shouldnt visably do anything, yet. Click Filter -> Blur -> Gaussian Blur. Play around with the level of blur and you'll see what happens, it makes the edges a little opaque. Play around with it till it looks right.
This is a good step to do before starting to mask, as then all the masking is is fine tuning.
One final thing of note in the dog head pic is a tiny bit of flow-controlled clone brush to continue some of the lines from the gun into the head.