The image reading library that the QubeGUI uses (wx.Image) supports a number of format, but not all and also not all flavors. There are known limitations for it viewing iff, some tiff flavors, and newer formats like exr.
To address this, the next version of Qube will have an external viewer option that will allow one to use almost any external image viewer like fcheck, framecycler, preview, etc. Stay tuned!