Thank you for your helpThe ClassicUO launcher and client are two separate programs.
The first bitmap is the client, which is open source. Look in the ClassicUO source code in the file src\Game\UI\Gumps\LoginGump.cs. Make changes and recompile.
The second bitmap is the launcher. The launcher part of the ClassicUO isn't open source, so there is no easy way to change any of that stuff in the 2nd bitmap. But you don't need to run the ClassicUO launcher to run the ClassicUO client as long as the client has a settings.json file to tell it where the client files are.
Hope this helps!