Hoi Adiraan,
About 1st question:
Indeed, this time you have to talk with tassos developer becuase that extension by default takes images only from Joomla Image tab, not Helix "Media Blog" tab. We cannot change that from our end, it must be supported by GSD itself.
About 2nd question, class name for Image. HTML code looks like that
<div class="article-full-image">
<img src="/PATH-to-FILE" alt="TEXT"> </div>
so there is no extra class for image only, and class ".article-full-image" is used around img.
In GSD Image tab you can also select "Custom Value" and put URL to image manually. For example
/images/2024/11/20/sea.jpg

Contact with Tassos Support how it can be solved in more smoother way.