If you're on Windows just use File Explorer: search for *.jpg/*.png, sort by date, then delete those files added within the last few days.
On Linux, run "find" with appropriate -ctime and -delete parameters.
As for your media library, it will now be associated with the artwork files you have just deleted, and each XBMC client will keep trying (and failing) to download these files into their texture cache. You're pretty buggered here, with only two options: 1) trash your media library and re-scrape, or 2) use the scripts in my sig to fix your media library - specifically, dump the details of your movie library into mklocal.py and have mklocal.py create the changes necessary to remove the now non-existent artwork from your media library.
Something like:
Code:
./texturecache.py jd movies | ./tools/mklocal.py --local /freenas/media --prefix nfs://192.168.0.3/mnt/share/media --artwork fanart poster clearlogo clearart --readonly --nokeep --output changes.dat
will process the specified artwork while mapping nfs://192.168.0.3/mnt/share/media to /freenas/media (and vice versa). Any artwork for a movie that cannot be found locally (or remotely) will be output to changes.dat with the value of "null" - when eventually set on the movie, this artwork reference will then be removed. This process will work for Windows, just specify an appropriate Windows path as your --local parameter with the corresponding root of your movie source as --prefix.
Once you have created changes.dat, pipe the details into "texturecache.py set" to actually begin the process of removing the now non-existent artwork from your media library (this requires a recent Gotham Alpha build or later):
Code:
cat changes.dat | ./texturecache.py set