How to center iframe in wordpress>

This has nothing to do with the iframe.
your main div has no width set.

change
<main id=”main” role=”main” class=”clearfix”>
to
<main id=”main” role=”main” class=”clearfix ” style=”width:100%”>

Best, Michael

Great, thank you!

Do you do custom work? If so, how much would you charge to center this and make it mobile optimized?

Yes I do custom work. centering is not a problem normally. But optimize this for mobile is the main challenge. So it depends on your requirements.

See https://www.tinywebgallery.com/en/about.php

Best, Michael