Another odd quirk of Rainbeam (which may be a bug), that I noticed from someone asking me a very similar question earlier, is that it seems to just ignore Markdown inside the <center> tag. So you'll have to use the HTML syntax instead:

<center><img src=https://example.com/image.png /></center>

I actually don't know much about Neospring or its server software Rainbeam, I just know a lot about Markdown and HTML, and there are tons of resources online for you to learn them too! (actually I've been meaning to look into how Rainbeam processes HTML ever since I learned it was a thing, but never got around to it...)