Hi Alexis,
I took the HTML snippet you posted here and pasted it into a Marketo email, then added an image URL to the token - and everything works fine.
Let me try to guess some reasons why that wouldn't work for you:
1. The image URL must be absolute and starting with http:// - like in

2. You can see the image only if you preview your asset. You won't see it in draft mode.
That's all I can think of without seeing all of your setup. Hope it helps.
Best,
Michael