You could also modify Opera's Accept header that it sends with https://chrome.google.com/webstore/detail/modify-header-value-http/cbdibdfhahmknbkkojljfncpnhmacdek.

You'd add a new rule in options:

Set the URL field value to * to cover all sites (set it to the site you want). Check the "Modify" box. In the Header Name field, enter Accept In the Header Value field, put text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9 Click the + to add the rule.

What that does is remove image/webp from Opera's original of:

text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9

so that the site doesn't think Opera supports webp. Sites can choose to ignore it, but this method has worked for some problem sites in the past.