The 1205 x 310 image in the Theme section of App Studio is used in the email that is sent to users when using the App Deploy functionality. When this image is updated, the app re-built and re-deployed the image in the email received may not change.

Some email clients cache images so even though the image has been updated in App Studio the image may not change in the deployment email.  In most instances the email needs to be refreshed to show the image.  In O365 OWA for instance right clicking on the email and selecting reload will show the new image.