Gimp

GIMP stands for Gnu Image Manipulating Program and that's exactly what it does.

Multiexposures

 * 1) Make sure the layers window is open (CTRL+L)
 * 2) Open your first image, it will be the background layer
 * 3) Add New Layer
 * 4) Open your second image, and paste it onto the first one, it will be the layer you just added
 * 5) highlight this layer and choose "Add Layer Mask" and choose "White (full opacity)
 * 6) You can now use your paint brush/eraser to merge the two images