Your mobo appears to have nine mounting holes - three lines of three holes each at top, middle and bottom. Ideally you will insert through each one a screw that turns into a standoff below the mobo. Note that each of those holes has a star-like pattern of metal "fingers" around it. Those are there to ensure that the mobo is grounded to the case at those points. BUT the mobo should NOT make contact with the case at any other point.
Now, inspect carefully how those nine holes match up with the standoffs under the mobo. If any does NOT match, you will need to make an adjustment. Take the mobo out. The case itself has more than nine holes in its base plate to adjust for different board designs. It usually comes with standoffs already installed in most of the holes. The classic standoff is a small brass piece about 10mm high with a threaded end sticking out (to screw into case base plate holes, and a threaded hole in the top to accept the screw that you put through the mobo mounting hole. You should have one standoff placed exactly under each mobo mounting hole. You MUST NOT have any standoff in any other hole because that would be a contact point that creates a ground to the case that should not happen. So review the standoff positions against the mobo holes and ensure they match.
There are some cases that do this differently. Some have used plastic standoffs and ensured mobo grounding in a different way. Some have provided not standoffs, but merely positioned raised parts of the back plate to accept the mounting screws, but then they have to provide some way to insulate a few of those raised bits from places they are not needed.
If you are finding that your mobo's holes pretty well lineup with the standoffs, but not exactly, they you need to shift the mobo slightly. That MIGHT mean that your I/O shield is not quite mounted correctly, or that the mounting plate attached to your mobo's back edge that contains all the external connectors is not properly lined up with the I/O shield. I am assuming you realize that the I/O shield is a separate (usually metal) plate that snaps into a hole in the back of the case. It is NOT attached to the mobo.