Celtic art is associated with the peoples known as Celts; those who spoke the Celtic languages in Europe from pre-history through to the modern period, as well as the art of ancient peoples whose language is uncertain, but have cultural and stylistic similarities with speakers of Celtic languages. Celtic art is a difficult term to define, covering a huge expanse of time, geography and cultures. A case has been made for artistic continuity in Europe from the Bronze Age, and indeed the preceding Neolithic age; however archaeologists generally use "Celtic" to refer to the culture of the European Iron Age from around 1000 BC onwards, until the conquest by the Roman Empire of most of the territory concerned, and art historians typically begin to talk about "Celtic art" only from the La Tene period (broadly 5th to 1st centuries BC) onwards. Early Celtic art is another term used for this period, stretching in Britain to about 150 AD.

From the 5th to the mid-9th centuries, the art of the Picts is primarily known through stone sculpture, and a smaller number of pieces of metalwork, often of very high quality; there are no known illuminated manuscripts. The Picts shared modern Scotland with a zone of Irish cultural influence on the west coast, including Iona, and the Anglo-Saxon kingdom of Northumbria to the south. After Christianization, Insular styles heavily influenced Pictish art, with interlace prominent in both metalwork and stones.  Pictish stones are assigned by scholars to 3 classes. Class I Pictish stones are unshaped standing stones incised with a series of about 35 symbols which include abstract designs (given descriptive names such as crescent and V-rod, double disc and Z-rod, 'flower' and so on by researchers); carvings of recognisable animals (bull, eagle, salmon, adder and others), as well as the Pictish Beast, and objects from daily life (a comb, a mirror). The symbols almost always occur in pairs, with in about one-third of cases the addition of the mirror, or mirror and comb, symbol, below the others. This is often taken to symbolise a woman. Apart from one or two outliers, these stones are found exclusively in north-east Scotland from the Firth of Forth to Shetland. Good examples include the Dunnichen and Aberlemno stones (Angus), and the Brandsbutt and Tillytarmont stones (Aberdeenshire).  Class II stones are shaped cross-slabs carved in relief, or in a combination of incision and relief, with a prominent cross on one, or in rare cases two, faces. The crosses are elaborately decorated with interlace, key-pattern or scrollwork, in the Insular style. On the secondary face of the stone, Pictish symbols appear, often themselves elaborately decorated, accompanied by figures of people (notably horsemen), animals both realistic and fantastic, and other scenes. Hunting scenes are common, Biblical motifs less so. The symbols often appear to 'label' one of the human figures. Scenes of battle or combat between men and fantastic beasts may be scenes from Pictish mythology. Good examples include slabs from Dunfallandy and Meigle (Perthshire), Aberlemno (Angus), Nigg, Shandwick and Hilton of Cadboll (Easter Ross).  Class III stones are in the Pictish style, but lack the characteristic symbols. Most are cross-slabs, though there are also recumbent stones with sockets for an inserted cross or small cross-slab (e.g. at Meigle, Perthshire). These stones may date largely to after the Scottish takeover of the Pictish kingdom in the mid 9th century. Examples include the sarcophagus and the large collection of cross-slabs at St Andrews (Fife).  The following museums have important collections of Pictish stones: Meigle (Perthshire), St Vigeans (Angus) and St Andrew's Cathedral (Fife) (all Historic Scotland), the Museum of Scotland, Edinburgh (which also exhibits almost all the major pieces of surviving Pictish metalwork), the Meffan Institute, Forfar (Angus), Inverness Museum, Groam House Museum, Rosemarkie and Tarbat Discovery Centre, Portmahomack (both Easter Ross) and Tankerness House Museum, Kirkwall, Orkney. Answer this question using a quote from the following article:

Where else?
the Meffan Institute, Forfar (Angus), Inverness Museum, Groam House Museum, Rosemarkie and Tarbat Discovery Centre,