Presuming it’s a solid graphics file with no transparent areas, here’s what you do:
1) Put the graphic file on a video track above all other content tracks.
2) Drag the duration out so that the still image is the same running time as your entire sequence.
3) Click on the graphic file to select it.
4) In the motion menu in “effects” select scale and reduce it to the size you want it.
5) Grab your file with your cursor and drag it to its position. Make sure keyframing is not selected – it will then be where you want it and stay there for the duration of your show.