Did you use an SD card or USB stick? I can't get mine to recognize the USB stick!
I'd been having the same issue lately, usb stick would show a circle with the slash through it (7000NEX, not 4100NEX) via USB stick but Ok via SD card.
Not sure if USB should be a viable transport vehicle.
EDIT: I take that back. I troubleshot it down to the CAMERA taking the pics. I have an older Canon elph (7MP to tell you how old) that any pic I take on it lets me use as a splash screen on my radio.
But if I try to use a pic taken with my iphone 6S+, the picture just has the circle with the slash through it when I try to use it as a splash page, like its in the wrong format or something. I resized it down to 800x480 but no dice.
I don't remember having this issue with my iPhone 5S.
Is there a tool that makes pictures "compatible" with the NEX headunits automatically?