It's hard to see in these reduced pictures, but the white sky at the left has a marquee around it. Larger white areas amonst the trees are selected too. Here are the directions I wrote for this method, using Photoshop Elements:

Put a blank layer over your background by clicking on the new layer
icon at the bottom of the layers palette. If you pull the layers palette
by the tab out of its parking place it won't close when you leave it.

Make a duplicate of your background by dragging the background layer
down to the new layer icon at the bottom of the layers palette and drag
it over the blank layer.Let go of the mouse button and it will be on
top. You will have three layers with the blank in the middle.

Click on the top layer (the duplicate) and use the Magic Wand at
perhaps 20% tolerance. Make sure there's a check in the box that says
"Contiguous". Otherwise the white stripes will be chosen too.
Click on the sky area. Hold down the shift key as you continue to use
the Magic Wand.

Some of the stripes may start to get added. You can Click on Edit/Undo
to reverse the last click.

Another thing you can do is switch to the lasso and hold down the alt
key as you draw around areas you don't want to add.

Holding the shift key as you use a tool adds to the selection. Holding
down the alt key deletes from the selection. To fill in tiny spaces
zoom in and change the tolerance to 5. Hit the capts lock key and your
cursor will turn to a plus...it allows for more accurate placement.
Click in the tiny white areas.

When you are finished selecting, making sure you have white as the
background color go to edit/cut.

Make the blank layer the active layer by clicking on it.

Click on the foreground color (black) so that the big palette of color
choices shows up and choose a nice greyish blue which will be used for
a gradient!

Click on the gradient tool and the first gradient on the top left of
the options bar will be a blue to clear box. You will click in the top
middle of your picture and holding down the button drag down until you
get into the tree area. Let go of the mouse button. You will have
darker blue on top and nearly white on the bottom so it blends in with the
little white spots in the trees.

Your selection may not be perfect and there are ways to make it better,
but I think you will know a lot more about Elements when you finish
this tut!

Save the image when you are satisfied as a PSD file to retain the
layers so you can work on it some more.

Then save it as a jpg.