I'm not sure if this fixes Mario's issue, but Ubooquity doesn't seem to like gray+alpha PNG files. For instance, if you `file` the PNG file in Linux and see this:
[abc@xyz ~]$ file tmp/089-044.pngtmp/089-044.png: PNG image data, 920 x 1300, 8-bit gray+alpha, non-interlaced
The page won't display in Ubooquity. The quick workaround is to open the file in MSPaint and then save it which removes all the alpha transparency and grayscale attributes:
[abc@xyz ~]$ file tmp/089-044.pngtmp/089-044.png: PNG image data, 920 x 1300, 8-bit/color RGBA, non-interlaced
EDIT: I'm not sure if that's a bug, or intended behavior.
Customer support service by UserEcho