This is a whole lot of effort to change the splash screen.
It’s just an SVG file contained in this file: splash_svg.cpp. You can use multiple programs to export SVG as a CPP file (I believe, I’m not going to walk you thought doing this part). Then just replace the contents of this file with your output.
You can always just disable the splash screen by making changes starting at line 233 in Window.cpp and either commenting those lines out or deleting the text.
Then just recompile EmulationStation with your custom changes.
Note: I do not condone this activity in anyway and I believe the splash screen (in EmulationStation) should remain intact, but I can understand the desire to create your own custom image or to remove it altogether (having had to do that for a DosBox project recently I found it to be a pain in the ass).