_ph_image_digest function cimg exception when alpha channel present

[Reubot-J]

Function throws the following exception with images that contain an alpha channel as it is not being treated as an RBG.

[CImg] *** CImgInstanceException *** [instance(316,324,1,4,0x7fbd3d307010,non-shared)] CImg<unsigned char>::RGBtoYCbCr(): Instance is not a RGB image. terminate called after throwing an instance of 'cimg_library::CImgInstanceException' what(): [instance(316,324,1,4,0x7fbd3d307010,non-shared)] CImg<unsigned char>::RGBtoYCbCr(): Instance is not a RGB image.