I have just uploaded Smart Photo Grid 10 into Xara Cloud.

You do not have the same controls to move or add images. The heading clearly states "Photo selected inside a [sic] inactive Photo grid [sic]"
You can click on an image and rotate, move or resize it.
The same click action allows you to Replace an individual image.

Xara Cloud provides some 30+ "dumb" Photo Grids, with which to build up a similar effect.
These Photo Grids are a collection of nested Groups of clipped photos that are not interactive.
They are next to unworkable and are clearly Xara's current fudge.

The workaround it to use a Xara Desktop application.

I am not experiencing XC frequent refreshes but if I have the same file open in Desktop and XC then a save is not the quickest dog in town (around 60 seconds for me).

Acorn