Making 2 pictures partially and gradually overlap?

Hi,
I would like to partially merge/overlap 2 pictures of a scenery/landscape. How can I get the pixels change gradually from one picture to another (at first on the left hand side 95% of the pixels come from the left side pic, 5 % from the right side pic, then 90%/10%, then 85%/15%, etc. until the picture has completely changed to the right side pic).

If this cannot be done, how could I make the pixels of one picture gradually disappear, showing just the background, whatever that might be (any colour, or another picture).

The gradient tool always seems to ask a starting colour and I haven't found out how to define that it should start from the picture and then fade little by little.

I hope the question wasn't too ambiguous,
Minna

It's a frequent question.

There are a few ways to do it, but I would start by creating a document with two layers, each consisting of one of your two images. With the top layer selected, use Layer/Add Layer Mask to attach a mask to the top image. (It doesn't really matter whether you choose "hide all" or "reveal all", but it will probably be easier to see what's happening if you choose "reveal all".)
With the Layer Mask selected, use a simple black/white gradient and sweep across the image. This will cause the layer mask to gradually fade the top image, producing - I hope - the effect you're after. Experiment with your gradient to tweak the appearance of the fade effect.