See if a button's script with go to field [container field] will pop up the menu. You may need to specify the select option and you may need to use Go To Portal Row [Last] to put the focus on the container field in the correct portal row.
I don't think you can insert a photo in a container field on a portal row.
You can use a button above the portal to grab the record id and go to the portal rows form layout and activate the camera there.
I've found something like:
go to form
go to field activate and maybe a step more causes the dialog leading to the camera to open.
Sadly, Go does not recognize that we would like to just snap a photo and not have to do all the xxxxx....