Why can't I add files to Images.xcassets in Xcode 5?
I had a similar issue when trying to add a PNG file as well. I tried Importing as well as drag & dropping, and it simply wouldn't work (wouldn't give me an error, either). For some reason, I had to ensure that the filename ended in lowercase ".png" as opposed to ".PNG".
Not sure why it happened, but simply renaming the extension to lowercase ".png" seemed to fix the problem.