Could not load the "" image referenced from a nib

You can do this to check it from your storyboard, or nib.Open it as Source Code:

Then "Find" the image name in it, see if it exists but already invalid in your project, probably you have deleted or changed its name, and now it doesn't seem valid.

I had the same issue and in my case there was an UIButton with an invalid image reference in the storyboard file. The reference wasn't empty so it was not trivial to spot by just searching for "" in the storyboard file. However, Xcode did show "Unknown image" for the button in the Background field instead of "Default Background Image". In the storyboard file it appeared as

<state key="normal" title="Use" backgroundImage="0E39AEA8-7F29-40B2-96B1-63B99047E8D5">

so perhaps grepping for backgroundImage and looking for non-familiar references will help find the cause.

I had this warning because of a UITabBarItem. In the attributes inspector for the field "Selected Image" I had a file listed that was also the value for the field "Image". Nevertheless, I received warnings until I made the "Selected Image" field blank.