Well, probably the most obvious way would be:
Open the both files, and then bring one to the front.
Go to File -> Dialogs -> Layers
In the layer dialog, press the lower left button to create a new layer.
New copy and paste the second image onto this top layer.
In the lyaer dialog, you can adjust the opacity down to make the picture underneath visible as well.
I don't really know the end effect you want here, but:
You can merge the layers together by right clicking the top (new) layer in the layer dialog, and selecting "Merge Down".
There are many filters available to try to blend things, such as the Gaussian Blur, or the more manual Smudge and Blur/Sharpen tools.
Hope that helps.
