The simplest way is to use grayscale CSS filter.
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In page’s Page HTML Header add style rule with id selector, let it be
grayscale, to be applied to nested <img> element.

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Set ID attribute of your Image element to
grayscale.

If you don’t see ID attribute on Image element, unhide it in Settings of your app as described here: How do you enable an ID attribute to a input box or any item - Need help - Bubble Forum.