Hi. What type of video connection are you using between the Wii and the receiver, and what type of video connection are you using between the receiver and your TV?
It might be an issue of the receiver not being able to convert from one to the other.
EDIT: On that receiver, no conversion between video connection types is made. So, a component video input must be sent out a component video output, and the same is true for a composite video input/output. For reference, that's shown on page 13 of the owner's manual.