Photo management on old MAC

Which app? iPhoto? Photos? Aperture?

In general though, you wold select all the images you want to delete and press the delete key or go to the image menu (photos app) and select delete from there. The exact keypress or menu item mechanism for deleting may be different in different apps, but whatever it is I would expect it to apply to all selected images.

Same as in anything. Select more than one at a time;

Click to Select

Command-Click to add to a selection

Shift-Click to select contiguous items

etc.