I've searched and found advice to use a GIMP layers dialogue in e.g. How do you get transparent colour on Gimp? However, in my version of GIMP (2.6.8 for Mac), Layers doesn't seem to be set up to display a dialogue. I've seen it in previous versions but hunted and not found anything under the Layers menu that will bring up the Layers dialogue.
How, in the present version of GIMP, can I set the opacity image-wide to 30% or 70%?
I've been using 2.8 for a while but most things are still similar to 2.6 --
The layers dialog can be shown from Windows->Dockable Dialogs->Layers.
The other option is the little popup menu to the right of every dialog (little square with a triangle in it). Click it and select Add Tab->Layers. Once a tab is added you can drag it out to a new dialog.