PSS Settings > Webstore (Manage Media)

Prior to trying to set any media (logo, banners, tiles) in PSS, you will need to first load the assets into PSS Media Center.  Once they are in Media Center you can then navigate to them from Webstore Settings by clicking "Add/Upload".  Make sure to click the blue "SAVE" button after each change.

Logo

For best results use a logo that is at least 4.5 times wider than it is tall.  100x450, 200x800, etc.

Banners

If you are using the Corporate Theme you will want a banner size of 1600x767.  If you are using the Classic Theme you will want a banner size of 1920x1080.

Tiles

If you are using the Corporate Theme you will want a tile size of 800x523.  If you are using the Classic Theme you will want a tile size of 1920x1080