There are some markdown versions that allow to specify the dimensions. This is not standard. But, if there was a markdown-it-enhanced-img or something, we could easily add it to Joplin.

Some allow something like:

![pic](https://example.com/pic.jpg =150x100)       
![pic](https://example.com/pic.jpg =150x)       # height not necessary, will be calculated

See also this article: