Users in the marketplace forums often post really professional looking promos for their Bitcoin-related site/service. How do they do this?
You need to have basic BBcode knowledge nothing else.
This is a well organized guide to use BBCodes by @xtraelv, it can help you further.
https://bitcointalk.org/index.php?topic=4738016.0The text in the message body isn't selectable which makes me think they're taking screenshots of their site, saving as a .png, and just uploading the image slices. Presumably there's more to it than that (or is there?).
You are exactly right! They are probably just uploading the images on third party image uploading platforms like
imrgu and using the BBcode to post it.
You can even change the image sizes displayed by indicating the height and width. Refer the guide above.
Any tips on the best way to craft such a post in terms of optimal image sizing, the mechanics of inserting the images, etc.? Is it simply a matter of repeatedly clicking the 'insert image' and uploading?
I would suggest, you could see others examples, use the quote button and check there used BBCode, you would get a better idea.