Subscribe to the campaign you are interested into and click on the button "promote it". Browse the creatives to find the one you want to use.
Copy the banner image and paste it in your image editor. Then click on resize and insert the value in pixels.
Please note that you can edit size and weight of the banner sets we provide you, but not change the look and feel of the banners.
You can use your custom creatives only after the approval of the campaign manager.