I can't see the image very clearly but it looks like you are not plugging anything into the 16X PCI-E slot.
Plug the the first card into the 16x slot using a 1x riser (It fits) just doesn't use all the space on the slot.

A PC generally looks for a graphics card on the 16x slot at boot to output to the monitor. You can disable onboard graphics/audio/com ports and all unnecessary devices in BIOS.
The cards must be powered as well as the Risers. The card will draw power from the Riser and the 6/8 pin power cables connected to it.

I could be wrong but the power to the MB near the PCI-e slots is not necessary when using powered risers. I think they are there if you use risers that are not powered.
I know on the Biostar TB-250 you dont have to plug in the molex power cables if you have powered risers.