a) Install the extension called ... "Install Chrome Extensions"
b) Search for "Gif Jam (Animation Stopper)" and install it from the external site, or use this link:
Opera crashed the first time I did that, but it worked second time around. Tenor is slow to load but gets there eventually, showing the first frame of the GIFs only. I've not tried any other GIF sites yet. Hopefully Opera will add this function to their settings, sooner or later.