I did further research into the resource macro story and this is what I found out:
all Platform stacks which deal with images on the html page work perfectly fine with resource macros. The issue that the image does not show up in edit mode seems to be common and not related to Platform.
However, if a resource macro is used for a background-image (CSS!), the macro is not expanded. Which seems to be neither a bug nor a feature and a similar behaviour has already been discussed in a thread on this forum (RW 8.1.3 Qube 5 custom background image), the workaround by isiah however only works for CSS which was provided in the Code section of the RW project.