Details? Sure, no problem. ^^
All was done in Photoshop CS3 (I have CS6 at home but this was just a quick edit at work), can also be done in older versions of Photoshop.
I'm sure this can also be done in
Gimp (which is completely free, yay!) although I'm not familiar with its interface.
I'm afraid programs like digital photo professional aren't going to cut it, they allow for color management and cropping but aren't made for actual edits.
1. Open the image and make a selection of the trunk at the right of the image. This is what we're going to move to the left to get rid of the OOF trunk.
2. Copy this selection into a new layer (ctrl+c, ctrl+v) and move it over the OOF trunk roughly to where you want it to be.
3. Add a layer mask to the new layer (button encircled in red, mask icon encircled in black).
Now, let me try and explain layer masks a bit as a lot of you might not know this feature.
A layer mask is basically a black-and-white image of the exact same size as the layer it is on.
The mask defines transparency in the layer it is on. A black pixel on the mask makes the same pixel on the normal layer completely invisible, while a white pixel on the mask leaves the same pixel on the normal layer completely visible.
Gray pixels result in half-transparent pixels (the darker the gray on the layer mask, the less visible the pixel on the normal layer).
We can use this by drawing on the layer mask with a simple brush. You can draw over the areas you want to make invisible with a black color. If you identically draw a bit too much you can simply correct it with a white brush.
Using a hard brush will result in sharp cut-outs, however that is not what we're looking for here. We're going to use a very soft brush which results in a nice faded edge.
4. Using a soft brush (like I mentioned above) hide the left edge of the trunk layer so it fades into your original image smoothly.
If you hold the Alt button on your keyboard and click on the layer mask Photoshop will show you the layer mask instead of your image, this can be helpful to see what you're doing (or in this case to see what a layer mask looks like).
It is possible to paint on the mask while in this mode, I don't really do it a lot though as layer masks are usually made while using the image as a reference (we're painting along the tree, thus using the tree as reference).
Just click the layer mask holding Alt again to get back to the image.
5. Now the edge of the trunk layer is faded you can move the layer into it's final position using the snow-line (encircled in red) as reference.
6. You can now refine the edges of the layer mask so everything falls into place.
I've encircled the two important points that need some extra work. The deers' leg should be fully visible of course and the second snow-line should continue right up against the trunk. You can do this using a very small brush.
7. The last step is simply cropping the image to get a nice composition and at the same time throw away the excess parts of the image.
Voila, we've got our image.
I hope this helps a bit.
Any questions are always welcome of course. ^^