John Gruber remarked that image syntax was his biggest mistake with Markdown, and mentioned an alternative:
My best idea for good Markdown img syntax would be to just paste in a URL ending in .jpg/.png/.gif etc.
In our own search of support for file transculsion we landed on a syntax for what we call, Content Blocks.
Adding Content Blocks
Embedding images, comma separated tables, text files and code as blocks of content in your documents is easy on Mac.
Simply drag these files into your Editor or type their path using the syntax below.
Section.txt "Section"
Balance Sheet.csv 'Finances'
images/Structure.jpg (Data Flow)
💡 In iA Writer 6.0+, autocomplete will help find and add Content Blocks for you. You can trigger autocomplete by typing a / and the first few letters of the file you’d like to add. Then, select and hit return and, voila!
Rules
- Content block references must begin at the start of an empty line
- You may specify a caption in quotes or parentheses (this will be ignored when used where it does not have an immediate purpose, such as with .txt files)
- Only files in the Library may be referenced (dragging a file onto the Editor adds a copy in the Library)
Exporting Documents with Content Blocks
Among many export options, iA Writer for Mac allows you to export an archive of a project containing Content Blocks.
When you export a document that includes Content Blocks, the blocks are exported to a .zip archive maintaining the structure of your document.
Learn more about export options!
Content Blocks 
