I want to end up with:
Hello there!
This is an image
Hi!
Where the image and the text This is an image
I think I have a simple solution that will work given that you can define CSS. It also does not require any extensions or HTML! First your markdown image code:

Note the added url hash #center.
Now add this rule in CSS:
img[src*='#center'] {
display: block;
margin: auto;
}
You should be able to use a url hash like this, almost like defining a class name.
To see this in action, check out my JSFiddle using SnarkDown to parse MarkDown in a textarea - https://jsfiddle.net/tremor/6s30e8vr/