Things You'll Need
Instructions
Load the Pokémon Fire Red ROM in UnLZ-GBA
Rip and save the needed files from the ROM using the following UnLZ numbers: 141 - Flames, 2009 - Pokemon Logo, 2010 - Pokemon Logo Tilemap, 2011 - Charizard, 2012 - Charizard Tilemap, 2013 - 'Press Start'/'Gamefreak,' and 2014 - 'Press Start'/'Gamefreak' Tilemap. These are the title screen items which can be edited.
Open whichever image you want to edit, the Source Image, using the tile map editor. Note its properties: length and width, color palette, etc.
Open the goal image in Microsoft Paint. This is the image with which you want to replace the Source Image.
Clear the Source Image from the tile map editor
Copy and paste the Goal Image from Microsoft Paint into the tile map editor
Save the Goal Image with the Source Image's original size and properties
Re-open the Pokémon Fire Red ROM in UnLZ-GBA and click the "Import" button
Select your newly saved image to import. Use the auto-selected settings
Save the Pokémon Fire Red ROM. The game will now start with your newly-edited title screen.