Download content from Sims 3 Store

lacy9265​  Sorry for the late reply.  When downloading a world, it often shows up as a generic icon first, as you're seeing, and only a few (5-30) seconds later changes to the proper thumbnail.  I've found that if I try to install the world in the interim, it fails, and other already-installed content can break too.

Anyway, please go into Documents > Electronic Arts > The Sims 3 and remove the Downloads folder.  Nothing in this folder is required once you've installed the files through the launcher, but you may want to keep backup copies of at least some files if you don't want to have to download them again.  Point is, remove the folder so the game creates a new, empty one.  Download Monte Vista and wait until the thumbnail changes to a preview of the world, then close the launcher and reopen it before installing.

If you don't see the installer, look for a second plumbob in your Dock, and click that to bring the installer's window to the front.  The window can take a few seconds to appear as well.