@KimS2727 - I used my sim's own artwork. I had her put her paintings in her inventory than I placed them on the wall.
Sometimes when you take them directly from the easel and hang them the game does not recognize them. I had her put up cheap ones first then replace each one when she painted a more expensive duplicate. She built up house worth that way without having to actually build. Of course her taxes went up.
@JENNIN - It is never too late. There is no time limit and you can even do the challenge more than once if you wish, trying to beat your own score or make the rules even harder.
@Shadow2Soul - Your house is awesome. What are the gold things in the front yard? I couldn't enlarge the pic enough to tell.