oh well thats a tough one ^^ but i´ll try
1st thing is of course the photo itselve this one works very well because the pullover actually has
the shape you would expect! this makes a huge difference in whether you "believe" the picture or not.
2nd thing is putting the pony behind the objects. I have 2 techniques and sometimes one is better sometimes the other.
a) would be cutting out the thing from the photo you want to put the pony behind. make this a new layer and now its very easy to put
the pony behind this object because you can change the "order" of the layers in the right hand "layer editor" window (small green arrows)
this enables you to move the pony vector just the way you like without cutting out the pony over and over again.
b) but sometimes its better to cut out the pony itself and place the different parts just the way you need them. con: when you mistakenly cut out the
pony the wrong way you need to start over^^ but it really depends on the kind of picture which technique is better
3rd thing is the shading and lighning effects. you can of course spend hours with trying to get the right shading manually (with some kind of "semi transparent"
brush) or a way faster method is to use the build in gimp tool you find in the top row point "filter" -> "light and shadow" -> "light effects"
this tool enables you to set in a calculated source of light which also renders shadows on its own. you can change the color of the light, the strenght, the distance, the relative position towords your pony. bit its important to have the "pony layer" selected because only the active layer is affected.
i hope i could help you at least a bit^^ but if you have any questions just ask