I'd second the advice given above - document all artwork properly, it's a right pain if you don't and something goes missing.
Also, make sure that valuations are updated regularly (I have a dim memory of every five years or so being suggested) and that this is done properly.
Anyone who stayed at RAF Henlow from the 60s to the 90s may recall the large paintings of the Taj Mahal that used to adorn one end of the Officers' Mess dining room. Every few years they were re-assessed for insurance purposes, with their identification as being by William Daniell and being worth a few Łk. Then, around about 15 years ago, a new and rather keener and more observant art expert was brought in to look at the paintings, who apparently got to the Taj Mahal pics and went "***ing hell, those aren't by William, they're by his uncle Thomas Daniell and worth about half a million quid!" Insurance costs meant they had to come down and got auctioned off, which is why Henlow was (and probably still is, unless all the money was in Icelandic banks) very well off as RAF messes go.
Henlow also has some David Shepherd pictures, in the form of small paintings of aircraft associated with the site. In case anyone wonders why they are so small, they were originally painted as place mats back in the early 60s and were used as such for several years until it dawned on someone that people were taking their soup off original Shepherds! Incidentally, when I was House Member there I got to read the artwork file and found that there were originally about 30 of them, but that a dozen went missing in the 1980s whilst in storage when the ante-rooms were being redecorated. Someone, somewhere, has a nice set of David Shepherd aircraft paintings hidden away...