What are 100096HTML firewall events

I’m getting hundreds and sometimes thousands of these events every day in my firewall section.

They are always on get requests for images on my website and have details related to yahoo in the match triggered section as below. Sometimes the ip addresses getting the images are yahoo ip addresses and sometimes not.

100096BASE_MATCHED_VAR=https://uk.images.search.yahoo.com/images/view;_ylt=awrj6txy0qhbfvoacily3olq;_ylu=x3odmtizmzexnm01bhnlywnzcgrzbgsdaw1nbg9pzamyndzhn2nkzwmzzwe3mdzkzwvhzdgzyjy3zwuyywu2zqrncg9zazqwbgl0a2jpbmc

Does anybody know what they are?

Thanks

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