I can’t speak specifically to Avelon because I don’t know how it handles (or if it even does handle) uploads, but for posing a picture on a third party site like tinypic, the markdown structure is:
![Alt Text](pictureurl.com)
That’s what I use in mlem.
Perfect, it’s frustrating when those features aren’t obvious, and it seems like wysiwyg posing would be table stakes for a good app after this long. I love mlem, but that’s a big part of why it’s my secondary browser and not my primary.