Images can be overlayed in M.App X if they are both georeferenced, conversely if two images are not both georeferenced then they cannot be overlayed. There are various reasons to overlay images including: virtual mosaicing, change detection, and layer comparisons. If both images are georefenced then they may be overlayed when using Map Space.
Image Space vs Map Space
When an image is collected by some sensor the relationship between the image coordinate and the coordinate on the ground from which it came is not a simple one, meaning that the relationship between the pixel coordinate and the ground coordinate is not linear. Typically there is a complex set of formulae (called the sensor model) which are used to derive a ground coordinate from the image coordinate. When an image is displayed, such that the relationship between the image coordinate and the screen coordinates is linear we say it is displayed in Image Space. This is what happens when "Display as Image" is used. The math model can be used to created an ortho corrected image whose pixels are related to the ground coordinate using a simple scale and offset (i.e. linear). When an image is displayed using such a transformation we say it is displayed in Map Space. The coordinate system used in Map Space is typically one of a set of well known geospatial coordinate systems, that are represented by an EPSG code. The are two ways to initially display an image in M.App X: Display as Image and Display as Map.
Below left - Image Space. Below right - Map Space.
Most WMS services are prepared to provide imagery in any number of geospatial coordinate systems, so it is possible to overlay two geospatial images when using Map Space. Display the first image using Display as Map, and this display the second using Display as Overlay. The transformations between different geospatial coordinate systems are well known and can be performed rapidly by the server as the images are being streamed to the client.
Two images overlayed as virtual mosaic.
The result of a slope computation displayed on top of the original terrain and then compared using layer opacity in the Contents panel..
Images from two different dates overlayed and then compared using the swipe tool in the contents panel. The swipe line is marked by the orange line and the area of change is indicated.
When an image source containing multiple images is used (such as NITF), it can be displayed using Display as Stack which will display the first image as map and then it will display all subsequent images in the file as overlay.