help with CSS?

thanks so much! the community here's been pretty nice so far

I'm struggling to figure out how to add an image, but to make something move with the screen set
position: fixed;
tusk69420 (https://myspace.windows93.net/?id=5765) has done something like this with a bob under their sidebar so you might be able to ask them! goblin_n_screen has done the same in the right bottom corner so you could ask them for help as well (https://myspace.windows93.net/?id=8328)
zoey (https://myspace.windows93.net/?id=2951) has added an overlay to images and has a similar computer

Just a bit, I’m trying to find out how to add image overlays and images that track with the screen

do you still need help ?