This gives you a 100% custom, unshared file that is guaranteed to work.
Create a .png or .jpg image to your desired size (usually 1024x1024 or 2048x2048 for best results). Convert to VTF: Use tools like VTFEdit plugin for Photoshop to convert your image into a .vtf file. Save the file as startup-loading.vtf Place the new file in your game's materials/console/ Where to Download Existing Customizations startup-loading.vtf download
Let me know which game or mod you're running! This gives you a 100% custom, unshared file