Octave will use GraphicsMagick for reading and writing your images (not plots, only if you are actually doing image processing). This means that the GraphicsMagick configuration you have on you system will limit what your Octave installation can do. Most systems will have it on their own repositories. Unfortunately, their configuration is not always the best for image analysis as it limits reading of images to 8-bit.
To solve the problem, GraphicsMagick needs to be rebuilt with the appropriate settings. See below for tracking dependencies and the reasoning behind each flag. Once done, the following commands should work to build GraphicsMagick.
./configure --with-quantum-depth=16 --enable-shared --disable-static --with-magick-plus-plus=yes make make check sudo make install