update Assign - error with optional type assignment

[heckj]

I reported the strange error as feedback, but also asked in the dev forms, why I wasn't able to assign() to image on UIImageView.

The answer was that the error was a type mismatch. It was attempting to assign UIImage to UIImage? (an optional), which caused the compiler to have it's freakout, reporting only: error: type of expression is ambiguous without more context

One option that was suggested was making a non-optional argument, and assigning into that.

In any case, it's good detail for updating the reference on assign() and how to use it