Please, can't users submit image banners on front end?
Only HTML code option shows on front end.
Please, not sure how many users can submit an HTML banner

HTML sounds like universal solution. With image it just bring problems. What should be limit, size, dimension, in what way to show it, ... All can be easily handled in HTML.

How can a regular user know HTML, @mbthemes?

lets not keep HTML, or Image..


Allow the user to contact us using the portal's button, but when submitting, he must answer our questions, such as the name of the business, person, or organization, other requirements etc.

Once we get a budget, we'll get in touch with him separately and design his banner. It's our website, so the quality of the banner is also important to us.

but here we need to maintain the threshold limit of the budget, let admin decide minimum budget user should deposit to us to use banner advertainment. 

@Ghany
Regular user no, advertiser yes. That is expectation.