I can't lock my reference image because there is no visible button for disabling the image to be editable:
Why doesn't the “Disable for viewport selection” button doesn't show up in the Outliner? [duplicate]
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Look in the top right of the Outliner tab (in the Default workspace, this appears in the top-right) for a sub-menu called "Restriction Toggles"; it has a symbol that looks a little bit like a triangular key on it. In that sub-menu, you can choose which Restriction Toggles to make visible. By default in 2.80, only Visibility is present, but in this sub-menu you can turn on/off the Selectability, Renderability, etc restrictions toggles as needed.
The Outliner tab appears here in the Default workspace:
Within the Outliner, can find the relevant sub-menu here:
Inside this menu, you can choose from several available Restriction Toggles that you either do or don't want to have appear in your Outliner:
