I'm trying to paint my models to match the artwork and/or movies, as far as possible.

Trouble is the available artwork usually only shows part of the character, as it has to fit into the Deployment card or onto a box:

Occasionally I will luck into finding the more complete picture:

...or a page like this where the artist, or FFG, or LucasArts share the artwork; but this is very much a matter of luck and even then, as with Han Solo above, isn't always particularly high resolution.

Is anyone aware of a link to a good resource for the artwork used in FFG's IA products specifically, or Star Wars more generally? (Better than Google Images, which is fine up to a point but doesn't always have it).

I don't know about the FFG images, but I often fell back on the 501st's standards for various "official" costuming notes. By "official" I mean that the character/mini is canon, not that the standards come from Lucasfilm/Disney. Sometimes I would look through other movie prop-related forums as well. Those guys from the 501st are dedicated ! In fact, when they needed a bunch of troopers for a shot near the end The Mandalorian's first season, that's where they went to get extras!

If I couldn't find anything from the 501st for a canon character/costume, I'd watch the movie to try to get an idea or two.

In the case of Chewbacca, it was one of the SW cartoons on Disney+ that showed me how his fur darkened in certain areas.

And you can sometimes find images for the Rebels characters that provide a color map for the character.