The problem is that the format you provided isn't part of a commonly used Markdown spec - Joplin follows (as closely as is practical) CommonMark with some GFM extensions which means implementing this would instantly make it non-portable.

It would probably be easier (and more portable) to use a plugin that can convert markdown image syntax to html syntax (not sure if one has already been made or not).

There is also an example plugin that already uses similar syntax to add image manipulation to markdown links - Joplin Plugins - Enhancement