Although Napoleon predated photography, there is little reason to doubt that his many official portraits were a reasonable facsimile of the great man. Napoleon's death mask survives and his gaunt face (Napoleon lost weight from his likely death of stomach cancer) matches the early portraits like Gros's painting of Napoleon at Arcola or his visage of the 20 Franc coin. Note in particular the comparison between the jawlines of the portraits and the deathmask and the aquiline nose in the Gros painting, they were quite similiar.

The important thing to realize with Napoleonic portraiture, and royal portraiture in general, is that while tidying up physical appearance was important, it was more important that these images conveyed a political message about the monarch. The main message of David's famous portrait Napoleon Crossing the Alps portrays Napoleon as a romantic figure continuing in the tradition of Hannibal and Charlemagne. The appearance of Napoleon such as his windswept hair, the apparent calmness when mounting a energized horse, gestures to the sky, all serve to create a picture of a man of destiny. Similarly, David's The Emperor Napoleon in his Study at the Tuileries uses conscious details to portray Napoleon as a responsible head of state. His desk in the painting is cluttered with work, including the rolled up Code Napoleon. The carpet is ruffled at the chair, which creates a sense that Napoleon is in the habit of getting up from his desk often, likely to consult the books on the left side of the painting. Note both the burned out candles and the clock passively imply that Napoleon was hard at work well into the night. His uniform is not ostentatious and relatively disheveled as is his hair But Napoleon's face, which matches the deathmask, is serene and hid hand in the waistcoat is a classical pose that as one 1738 book on etiquette suggested "manly boldness tempered with modesty."

What these portraits show is that alterations in reality in Napoleonic portraiture were not focused so much upon physical appearance but upon fixing Napoleon into the great events of the time. Napoleon's crossing of the Alps in 1800 was much more prosaic and instead of a glorious white horse, he crossed on more sensible choice, a mule. The process of writing the Code was a committee effort and not the sole input of one man. Because these portraits of Napoleon served the interests of the state, Napoleon took great interest in preserving the homogeneity of his image and their overall accuracy.

Generally speaking there's little reason why they wouldn't be. Napoleon took men of Science, Art and Culture with him on his Egyptian campaign and many of these guys, flattered by the offer, painted favourable views both of the campaign/battles (of which most were a disaster after the Battle of the Pyramids) and of Napoleon himself.

That being said, we needn't look at paintings of the time as being inaccurate: We have to understand that these guys in many cases devoted their entire lives to painting portraits and sculpting busts.

With regards to Napoleon himself the sheer scale of artistic impressions gives us a fairly decent idea of what the man must have looked like. During his reign he regularly decided on the artistic direction of Paris by banning books and deciding the topic for arts festivals. During his reign there was an explosion of paintings, cartoons, statues and busts: The sheer volume of which has made a pretty coherent image of the man who was.

Sources: Andrew Roberts: Napoleon The Great

Alan Forest: Napoleon

If they weren't how would we know? He died in 1821, unfortunately just a few years before the camera was invented in 1825.