I wish I could do this, or even leave something at my desk for the next day. But with agile seating I don't even get a cubicle to myself anymore! Awesome! So motivated to be productive!
To be fair, if it's my personal office space that I have to put up with 40-60 hours per week, you're damn right I want it to feel homely. I'd have me some nice slippers under the desk, too.
Space heaters are limited to 1500 watts, which runs on average .20/hr. If they are run constantly while the employee is present (8 hours/day, but not likely) then it would cost the employer $32/mo. Granted, a non-zero number of employers are stingy enough to make this an issue, but realistically, it's not a big deal. Also they have safety systems in place which make them not particularly hazardous. Most have thermal fuses that burn out/disconnect power at temperatures significantly below what is needed to start a fire.
Space heater is never a cost issue, it's an issue because it gets plugged into a circuit with a dozen office PC's, which will now trip constantly. And some people like to aim them at their feet next to the office PC and melt the case.
The wallpaper could be a large piece of canvas that's held up with pushpins? Or maybe its patterned cloth and you can still pin things up. Why stop at a mini fireplace?
You can also donate surplus to elementary school teachers and so they can bulletin boards look nice. Donate because they are on thin budgets already, but you can justify it to the board with something like "getting future customers familiar with our service early on"