The PGF backend uses \pgfimage

to include raster images (e.g. drawn by imshow). This does not work with \subimport in Latex, whereas \includegraphics does.

According to https://tex.stackexchange.com/questions/32986/, \includegraphics is to be prefered anyways, and its only drawback (the missing interpolate option) has been resolved in 2010.

If desired, I can create a pull request.