Go to "[your directory]:\SteamLibrary\steamapps\common\Tag The Power of Paint\Assets\Textures" just by clicking right mouse button on the game in your library -> properties -> installed files -> browse local files then clicking on the assets folder and going to the textures folder. Now you can drag any photo (png recommended; gifs don't work) or use any preexisting texture. In my case it would be spacecore.png because why not as Cave Johnson...

Go to "[your directory]:\SteamLibrary\steamapps\common\Tag The Power of Paint\Game Data\Level Definitions" (as I wrote earlier). Open any .xml file there with notepad. In my case it would be main menu file because it doesn't really matter for this guide.

Go to the top menu bar and then click on edit -> replace...
In the first one type "</Texture>" and in the second one type nothing so you can click "replace all" (it just removes something that'd be redundant as you'll see later)
Later in the first one type "<Texture>" and in the second one type "<Texture>[your_texture]</Texture> " (don't forget about that space at the end so it will just set the text of the previous texture apart from the commands)

yes, don't forget to do so with your .xml so you can check it in the game

(example I showed you)
(if it doesn't make sense to you it's that elephant from VVVVVV)
(preexisting texture changing gravity)