Your designers are correct. If you think about it for a minute, the proportions of each size are different so you can’t resize your aspect ratio without squishing or stretching the content. For example, to change the Eventbrite pixel dimensions to the Meetup size, you could adjust the width to 1200 but the height would be 594, not the 675 pixels you need. A quick illustration of these sizes shows how the content will have to be adjusted for each new size. You can also crop or extend the background as suggested below!