Is the picture above the perfect book cover? Simple, attractive, and pointing towards the book's role as a work of combat, primarily against religion (but with a few jabs at what Voltaire viewed as some of the less important areas of philosophy). Certainly far better than the stodgy hardbacks which fill up every shelf of academic libraries. Far better than any of my textbooks, past and present, whose pictures seem to compete for which can be least relevant to the actual topic.
One of the jabs Voltaire makes is at the study of Aesthetics. His view can roughly be summarised as, "Of course beauty is subjective, morons!" Quoting from the article entitled "Beau, Beauté" (translation by John Fletcher, from the edition linked to above):
I was at the theatre one day with a philosopher: 'How beautiful this tragedy is!' he said. 'What's so beautiful about it?' I asked. 'It's that the dramatist has achieved his aim,' he replied. The next day he took some medicine that did him good. 'It achieved its aim,' I said, 'what beautiful medicine!' He realised that a remedy cannot be said to be beautiful, and that to apply the word 'beauty' to something, it must arouse our admiration and give us pleasure. He agreed that this tragedy had inspired both feelings in him, and that was to kalon, the beautiful.
I would suggest that Voltaire's companion's idea of beauty as "achieving its aim" is in fact more valuable than either of them realise. The picture of Voltaire wielding his quill in a duelling position achieves the aim of indicating what the book is about. Another cover which achieves this aim is that of my copy of Ernest Gellner's Nations and Nationalism. I picked it up in a second-hand-book shop and cannot find an exact replica, nor any copy of the painting adorning it, but it is quite similar to this:
A cart of aristocrats rides through a torchlit public square, surrounded by cheering crowds. Flags - I presume Italian, though possibly French - are festooned everywhere. It is less simple than the Pocket Philosophical Dictionary cover, but is equally attractive and is so nationalist that can almost hear the trumpets just by looking at it. Sometimes using a classical painting might be interpreted as rather pretentious but this is a sufficiently weighty topic to merit it. Compare it to a more recent cover of the same book:
This is plain ugly. Admittedly nationalism is pretty ugly too, but given that Gellner seems to be advocating it this cover seems rather out of place. Moreover, what is inherently nationalist about building projects? Perhaps this aims to show the role of industrialisation in promoting nationalism, but there are more obvious ways of doing this.