Wallpapers? Do you mean the screenshots in the download section?
If you want to embed them into a forum post, just use the image URL (like https://www.sinsofasolarempire.com/images/screenshots/Entrenchment1_large.jpg or _small.jpg).

If you use on as a desktop background, open it in your browser, right click and select 'use image as wallpaper/desktop background'/etc.'
