I noticed the Ning Creators app banner size is 500 x 85 pixels and displays on the mobile URL at 253 x 40. I'm trying to configure my network's banner so that it looks as good as it can which is difficult for me because I actually don't have a smartphone to check this on, so my only testing ground is the preview I get on the Mobile interface in Ning Labs from the management area of my network. Can someone tell me what the optimal size is for the header image so I can get it to look good. I tried sizing it the same as the Creators but it had extra white space on the right side.
The width will vary by device, so there's no perfect size, but a height of 40px is optimal. Ideally you could pair the logo background with the background color, so that the logo doesn't clearly end and will look good on devices that have longer widths. FWIW, your current banner looks great on the iPhone and on a Samsung Galaxy 2S.