Once you have added some illustrations and contents to your figure, you may want to save it, right?
On the upper left side, you'll see the button "save"
It looks like this:
Just click "save" and it's done.
Importantly: the thumbnail will just be created once you download the figure.
So, if you please, after saving you can click "Download" to get a bitmap copy of your file.
In case of any doubt, don't hesitate to write to firstname.lastname@example.org