Thanks to Dan for moving this post.
Laszlo, this is not the correct method of aligning two rasters. If they are in different spatial reference systems, importing from A into B will not do what you are intending.
If they are both properly georeferenced (even in different spatial reference systems) and you view them in ArcGIS Desktop, the software should reproject either or both images into the spatial reference system of the MAP, and they should automatically line up.
Presuming you loaded them into the map and they did not align, something must be missing. If you have georeferencing information (metadata) for the images but that is not in a digital format ArcGIS understands, you'll have to edit that metadata. If either or both images has no georeferencing, you can use the georeferencing tools to align one to the other.
Please tell us in greater detail what data you have, what happens when you view it in ArcGIS Desktop, and what you have tried (specifically which tools or functions). Also let us know which version of the software you are using.
Cody