Hi Glenys,
When you add the images to the Resources folder of your theme project you should be able to find these images when selectin an icon or image from the image browser dialog.
The live test should automatically use the new pictures when restarting a live test.
I have validated in the most recent release (8.5.6). When this does not work for you, perhaps there is something differently configured. In that case it will make sense to create a support case and we can take a look what the cause of the problem is.
Regards,
Roger.