From Xojo Documentation

Property (As WebPicture )
aWebMenuItem.Icon = newWebPictureValue
WebPictureValue = aWebMenuItem.Icon

New in 2020r1

Supported for all project types and targets.

A picture that is assigned to the WebMenuItem.

Sample Code

The following code is in the Opening event of a WebPage. The icon appears to the left of the menu item's text.

hobbitsMenu.Icon = Frodo