Hi Bozo,
I'm surprised no one has answered you before?!
The problem is your 120+ planes.  CFS2 MB has an inbuilt limit of around 90-100 planes.  When this limit is reached Mission Builder fails.  This is the probable reason.  The failing to load images on starting is part of this, although it could also be some scenery MB does not like.
What I do is have several installs of CFS2.  I keep a standard untouched version of CFS2, usually the original install.
I then use others for flying, adding etc etc.  MB will then be free to use. 
If you wish to edit missions that include other scenery/locations you can either 'add' that scenery to the standard install, also a copy of the aircraft.  When finished and having saved the mission to your 'flying' installation version, you can delete the aircraft, leave the scenery(if you want).  This way you keep the additional aircraft down to a minimum and MB still works.
Alternatively you can direct (through MB properties) MB to use other installs as it's data base, just keep that magic aircraft figure below 90.
I agree a re-install would mess everything up and be a real pain in the %*$!  Either leave as-is and not have MB or go for a fresh install and direct the location of the installation somewhere other than default (if this is where your CFS2 that you are using is located).
You will then have to direct MB to that installation(MB properties), as it always uses the default location of CFS2 as it data base.  While you're about it, make several installations ie European, Mediteranean, Modern, Experimental etc etc.
Sorry this so long winded but it's one of the most common probs with CFS2, I think MS did not expect in 1998 or so for it to be so popular with the builders!
Hope this helps, good luck

Cheers    Shessi   

Thanks.