Is there a way to change that background image to something else and custom?

No, there is not. At least, not an official way that doesn't involve hacking the IDE's executable files.

I read about toolsapi. Is this helpful?

No. ToolsAPI is the IDE's native API which allows authors to write plugins for the IDE itself. The ToolsAPI allows custom items to be displayed in the splash screen and about box, but changing the splash screen's background is not part of the ToolsAPI.