I see Wizards has several "Art Packs" for use with products, but the descriptions aren't super helpful and they seem to be zip files of images, with no index.  Before I go downloading all of them and sifting through, is there an online index, or even gallery, of the images provided somewhere?

There's not an index that I'm aware of. The free art collections are actually pretty lame. I'd advise looking at some the paid art collections on the DMs Guild; some folks have put some decent stuff out there. I have used some of it myself in my DMs Guild releases.

Thanks!  

I'm going to take a different angle on this. The art packs from WotC are absolutely worth sifting through as there are some gems and usable art in them. Plus they are free.

There is a directory someone put together. It's a buck but might be worth it for time saved. Search: DMs Guild Creator Resource Art Directory

over time, you may find that renaming some of the files and/or adding a few more folders will help a lot. Considering your work is unique to you and only you know what you want, going through them is the only way to do it imo...using someone else's directory/classification system restricts you to their definition of what they're trying to do.

It is absolutely worth taking the time to download and sift through it. Particularly if you're familiar with the 4e and 5e sources they came from, because you can look up the stats and convert them to 5e. All of my products use the creator resources extensively.