Images (that are used in a splash screen) Not Appearing In Unity Build
This post details an issue I faced in Unity 6000.1.17f1 regarding the use of images that are used in a splash screen and a UI document, not appearing in a game build.
Images (that are used in a splash screen) Not Appearing In Unity Build
This post details an issue that I encountered in Unity 6000.1.17f1.
The Problem
I had a problem where re-using an image in a UI document that was also used as a splash screen image makes the image not show up in a game build.
I also had used the same image in a UI document that was in a scene that wasn't loaded at build index 0, and the image showed in the build. Changing that scene to be build index 0 made the image stop showing up in a build. The conclusion is that this issue only occurs in the scene with build index 0.
The Solution
I fixed this by importing the image into the Unity project again, and ensuring there's two copies of it - one that's used in the splash screen, and another that's purely for UI use in any UI documents that appear in scene build index 0.

Reflection
I do not know why this happens, but I wanted to share my experience in case it is helpful to anyone experiencing the same issue.