Making the upload files available at the time Hugo is running isn’t possible right now. Sometimes the images are there, for example when you click the “Rebuild” button on the logs page it brings everything together including all images. But as an optimization, Micro.blog frequently recreates the Markdown files and runs with no images present, since they have already been uploaded to the static servers that host the blogs.
If anyone’s curious, I wrote a blog post last year about some of this: Manton Reece - How does Micro.blog even work?
It’s a little out of date (things are even better now!) but long story short, we currently run Hugo on 3 servers and randomly pick one, so there would be overhead to having all images on all servers at all times for all users. Mounting from a remote server would also potentially create another single point of failure which we try to avoid. This might be too much information, but maybe it helps explain why it currently works this way. 
Back to the real issue… When an image is uploaded, we do store the width/height in a separate database, so we have the data. The Micro.blog apps also usually include width/height attributes in HTML.
One thought I had is that I assume this data is most valuable for the “Photos” page, so perhaps more metadata could be made available to that page only, without affecting much else.
