Hello! I jumped into your account and it looks like the template you're currently using was setup using the save as template feature which was originally based on the "Example Campaign" template. That template's default placement for the header image is left aligned, so there is not an option to center this. You could however insert a larger image (width-wise) which gives the appearance of being centered.
I would normally direct you to having the template tailored by our design team as they can definitely tailor any of our ready-made templates to your desired styling (brand's colors, logo, etc), but that "Example Campaign" template was actually retired quite a few months ago. With that being said I would suggest looking at some of our other ready-made templates to use and see if any catch your eye. One of the biggest reasons being updated HTML code, but also the fact that all of our current ready-made templates are mobile optimized.
Hopefully this helps, but please let me know if I can assist further.