You could use this example img html for your custom image inserts.
(its the same as what was previously linked to)

EXAMPLE:
Code
<img src="image.jpg" alt="EXAMPLE.com Image" style="width:1050px; max-width:100%; height:auto;">

If they are ad banners served from a third-party such as Google Adsense, update your code to their responsive ad unit
https://support.google.com/adsense/answer/3213689?hl=en

current developer of UBB.threads php forum software
current release: UBB.threads 8.0.1 // wip: UBB.threads 8.1.0
isaac @ id242.com // my forum @ CelicaHobby.com