There are different resolutions and dimensions on the end user's device being used and that is why the dimension that we highly recommend is at 1920x1200 because that should adjust to any device every end-user is using to view your campaign.
We also suggest checking this out for more information about which best image sizes or dimensions to use: https://www.webmalama.com/the-best-full-screen-background-image-sizes-for-web-design/
How to get your support tickets resolved faster
Statistics have shown us that the more you tell us when you report an issue, the faster we solve that ticket. So...
1 - If you have screengrabs, please include them. A picture is worth a thousand words.
2 - If you have recorded a video showing the issue you're having, please include that as well. A video is worth a million words.
We recommend a free service called Loom for creating these videos. (this is a link to useloom.com)
3 - To speed up ticket resolution, follow these useful tips How to Write the Perfect Support Ticket (That Gets Results Fast)