Hi egosophist
I added in this file /templates/ja_social_ii//local/css/custom.css your code and as you can see opacity for image is changed
I disabled development mode from template -> Edit style -> General tab , this enable the local feature that allows to add customizations directly into css and not in less files.
Regarding masshead background image, it is very simple to manage
If you open your ja_masthead module , published in masthead position you will see a tab named Configuration Conditions
in that tab you can add your specific configuration , let's take the example of contact us page
If you look at the menu this page is contained inside your main menu and it has as id value 253
Now look at the Configuration Conditions for item id = 253
That means "In contact us page ( id = 253 ) insert background image stored in images/joomlart/bg-masthead/bg-3.jpg"
So you can change background image and description for each menu item identified with a given id number
Hope it helps.
Regards
