It's the background image for the Overgrowth theme in Acmlmboard 2.5, an old niche forum software. Here is a preview of the theme on one of the remaining boards running it: https://board.kafuka.org/?theme=overgrowth

You can also find it as the background image in the retro zone of my website, which is made to look like said theme from Acmlmboard: https://voxelmanip.se/retro/ I thought it would be very fitting to use as the wallpaper when I wanted to take the screenshot. :)