Thanks to Carlos Martins at Internet's Best Secrets, a number of keywords were found that will link images to Google Calendar.
Also a huge thanks to Tored from the Kustom Discord server for all their help with getting this working.
I wanted to make use of these calendar header icons in KLWP so created this repository to track any images I find. If anyone wishes to contribute feel free to open a PR if you find more images!

If you're wanting to access the URL directly, the name of the image from images folder should be appended to the end of this URL:
https://ssl.gstatic.com/tmly/f8944938hffheth4ew890ht4i8/flairs/xxhdpi/img_{here}.jpg

For example, if I'm wanting to access the image for theateropera.jpg I would use the below URL:
https://ssl.gstatic.com/tmly/f8944938hffheth4ew890ht4i8/flairs/xxhdpi/img_theateropera.jpg

I checked ~130,000 words against the URL and downloaded any which worked so the images folder is a bit of a dictionary for what exists.
There may be more that weren't listed on the Internet's Best Secrets site either in the OP or in the comments, that are conjugations of words (e.g. americanfootball), but I've found and added as many as I've been able to.

To use this in KLWP for calendar widgets, all that's required is a single global variable.

1. Create a Global text variable called caltags (or similar)
2. Copy all the content from keywords.txt into the text variable exactly as it shows on that link
3. On your shape, set the bitmap source to be the following formula:

https://ssl.gstatic.com/tmly/f8944938hffheth4ew890ht4i8/flairs/xxhdpi/img_$tc(reg, fl(0,0,0, tc(reg, tc(reg, gv(caltags), "\s*,\s*", "|"), "(.*):(.*)", "if(tc(low,ci(title, 0)) ~= ($2), $1 + @) +") + @), "@.*", "")$.jpg

What this will do is parse out the gv caltags to find each tag, and as soon as it finds the first match, it will return the key for that, and fetch the image from the URl.

If you also want to use the Location images from Google Maps, feel free to check out the Wiki which has a guide on setting this up.