substraction of two images

Your experiment contains two images of the following resolutions: 

ImagePlot_156_sum (200,601) and

ImagePlot_159_sum (200,294). 

There is no obvious automatic way to subtract the two images without additional information.  For example, if ImagePlot_156_sum is just a higher-resolution sampling of ImagePlot_159_sum or if they both have the same resolution but cover a different extent of the field.

If both images cover the same spatial range you can use ImageInterpolate to interpolate one or both of them onto a common size and then you can simply subtract the two waves.  If the two images do not cover the same spatial range you need to extract the common range and create two waves of the same dimensions (which you can then subtract).

A.G.