Go to the project explorer and select a project. Click Form Phases. At the bottom there is a section called “Copying Package Resources”. I suspect that you will find two images.xcassets files there, and if so, delete them by highlighting and clicking the minus icon (there are legitimate reasons to have two or more, but you would purposefully do this so that it is outside the scope of this response). For the rest, add your images to LaunchImage.
The reason for adding two is unclear, so I’ll add that if you use multiple targets, you may have different images.xcassets for each target (by default, xcode adds multiple values). However, if most of the assets are the same and you do not want to control the multipliers, you can use them for all purposes and write an add script to create phases for copying various assets, such as the startup image, at compilation time. This is also done in the build steps: add a phase that runs the script and then a copy (cp) to copy the images. Needless to say, you have to do this with very few assets: I use it to launch images and application icons.
Edited to add: there is a new scheme in which startup images are in storyboards and / or xib. I assume that you are not using it because you said that you are using old startup images, but if you want, just put your launcher image in the / xib storyboard launcher image file.