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