-repage is used to position images, or remove the position of the images relative to a virtual (make believe) canvas. The + form removes or resets that information. See IM Examples, Page, Repage, and the Virtual Canvas
http://www.imagemagick.org/Usage/basics/#page
-flatten will place images on a 'filled in' version of that virtual canvas (starting at 0 to the size found on the first image)
-mosiac will expand that canvas in positive directions if nessary, but images still needs to be given an appropriate offset.
-layers merge (the new kid on the block) starts with a 'minimal canvas based on the real part of the first image, and extend it as necessary to cover all the images in all directions positive and negative. any existing virtual canvas setting is ignored. You can then remove any negative offset.
http://www.imagemagick.org/Usage/layers/#merge
See... IM examples, Layering Examples
http://www.imagemagick.org/Usage/layers/#layer_examples
In all cases you need to specify the position of images.
Another example of 'overlapping images' in in Photo handling
http://www.imagemagick.org/Usage/photos/#overlap
Note in the last example how we add the transparency to one image only, and the use -repage to position the image on the virtual canavs.