Blender 3D Cursor & pivot point of objects

Shortcut to move the 3D cursor to the origin Shift C

Menu N to bring up sidepanel or N bar then View Tab 3D Cursor

To change the pivot point of an object:

1. Select the mesh
2. Enter Edit Mode
3. Select the vertex where the pivot point is to go
4. Goto Mesh -> Snap -> Cursor to selected
5. Back to Object Mode
6. Object -> Set Origin -> Origin to 3D Cursor

To change the origin of a reference object :

1. Select the image
2. Set the offset X & Y properties to 0