You can add your logo to all of your campaigns by uploading it in the Account page of your account.
Go to this page by clicking the Account link from the top menu, then click on the Choose File button and upload your logo.
Remember to click Save at the bottom of the page!
Your logo should be a PNG file with transparent background around 200 pixels large at 144 DPI (72 DPI is fine too). For best results use a white logo (though it depends on your hero image, sometimes black or another colour will work better, however generally speaking white works best on most images).
Also a square aspect ratio (as wide as it is high) works best as we limit the width and rectangular logos can sometimes be a bit small on mobile devices.