Although RPG art has got more colourful and glossier over the years (mostly as publishers have got more professional and colour publishing, whether in print or PDF has got easier and cheaper), I’m not convinced that it’s got better.
Some of my favourite pieces of art (I use that term deliberately) from old RPG works are simple black and white line drawings.
Traveller has particularly good illustrations, my favourite of which is this ‘mercenary striker’ from Book 4: Mercenary. Dick Hentz is the artist.

Liz Danforth also illustrated for Traveller, but I think her best work was for ICE’s Middle-Earth Role-Playing, in particular the portraits she did for the ‘Lords of Middle-Earth Series’.

That’s really only the tip of the iceberg for Liz Danforth, a truly multi-talented individual. Look her up on wikipedia and you’ll be amazed by how many different things she’s good at.
RPG illustrations also seem to have got more uniform and perhaps more serious than they used to be, which is really the purpose for this post. Sure, the 5th edition artwork is uniformly excellent, but it’s all very coherent. 1st edition had a variety of styles, from the dramatic…

…to the elegant…

…to the downright silly…

No, there never was a Papers & Paychecks Roleplaying Game, although I think there should have been. Happy April Fool’s Day.