Either
1. Resize your sprite to fit the ScreenExtents. This won't be pixel perfect, or even have square pixels for obvious reasons.
or
2. Create a second tk2dCamera that holds the background (use layer masks to make it only draw in there) with override set to StretchToFit. This will stretch the background to fit the screen, but won't be pixel perfect or have square pixels for obvious reasons.
or
3. Make your background sprite bigger to fit all the aspect ratios you care about, so when it draws more it has something to draw.