为什么我不能添加文件到图像。在Xcode中xcassets 5 ?

When trying to add image files to Images.xcassets, there are certain images that I cannot add. I've tried both dragging-and-dropping as well as using "+" followed by "Import". In the drag-and-drop case, nothing happens. In the Import case, the files I'd like to import are grayed out and cannot be selected.When trying to add image files to Images.xcasse