This is only available with certain subscriptions.
In Gateway OMS you have the ability to upload your own logo so that it appears at the top of all of your delivery notes / job sheets
You first need a logo that is 400 x 200px and in PNG format. You can then upload it to your company in the OMS by going to Companies>[Search Company by GUID or Name]>Edit>Assets.
In the Assets section of the company, use the Choose File button that's next to Upload Logo and select your logo. You then just need to click Save.
If you use multiple suppliers on your site, you would probably want to do this for each of the companies that have been set up for this.