Never mind all of these latest posts. I managed to solve it myself. 
I looked through some of the issues posted on GitHub regarding batch imports, and realized that I can use <supplementary_file>-element for importing images.
I then imported a single issue with a single article, that also imported my HTML-file. From there I added galleys manually from inside the submission section, uploaded my images and checked that my article was correctly presented.
When everything checked out ok, I exported all of it into an XML file, in which I edited out the <embed nase 64> elements for -elements referencing files on the server instead.
Now my import works as intended, and I will now use this XML as a base for my XSD when creating batch XML for import.
If anyone is interested, I can send the XML-file or link it here for later reference.
Thanx for your help!