Is there a definitive explanation for generating and importing raw heightmaps?

I have been trying to update the heightmap on one of my terrains for hours. It is the first time I have tried to do this since upgrading from unity 3.5.7 to 4.2.1. The method I used to generate heightmaps using 3.5.7 doesn't seem to create something that will work under 4.2.1. My basic process is I have a perl script that extracts a numeric raster of elevation values from a large elevation data set, then, in Matlab I use those values to create a gray scale image of the data. I then pull that image into Photoshop, scale it to the proper size for the terrain, flip it as necessary for raw import to unity, and save in raw format. Previously this worked fine. The generated images look just like they always have. But, now instead of getting sizes in the import window they are reported as width and height of 1, though the files are not near 0 size and can be read back into Photoshop.

I have seen mentions of defining maps to be x+1, z+1 relative to the terrain x, z size. This didn't seem necessary under 3.5.7 and I just defined the maps to be the same size as the terrain. I have tried the +1 sizes with 4.2.1 and when I import the map I get an error that an array is not large enough.

I've hit about every permutation I can think of in trying to import a new heightmap, (including trying to save the raw file out in a program other than Photoshop, XnView (which BTW worked under 3.5.7)). But, no joy. Any heightmap pointers would be much appreciated!