Another blog requested by the paperboy.
Layer masks are a great way to blend images and do local tonal range adjustments.
If you have an image open just drag the background layer down to the create new layer icon in the layers pallete to make a copy of it. Now if you look at the bottom of the layers pallete, the third icon from the left is "create layer mask".
Click this and you will see the mask appear on that layer in the layers pallete.
You now use a paint brush with black ink to hide that layer that the mask is on.
If you make a mistake with the black just paint it back in with white ink. Obviously if the bottom layer is the same as the top this mask will acomplish nothing. Select the bottom layer and do a curve adjustment on it. Now when you paint with black in the mask you will see the underlying layer show through.
Remember that you can use a brush with a low opacity to show through lightly.
You can also apply a gausian blur filter to the mask to blur it.