Couple of issues around using images of mockups as masters

I am using export to PNG, as suggested in the docs, to generate mockup images that I can use as "masters." I am seeing a couple of small issues with that feature:

1. Image borders: for some reason, exported PNGs have a white border around them, which makes it difficult to position several masters near one another (the white boarder of one master overlaps the other master).

2. Non-transparent background: if it were possible to set the transparency color on an imported image or if the mockup PNGs were automatically exported with the transparent background, it would not be necessary to carefully devise the stacking order to put one master inside another one. Also, #1 above would not have been an issue, though it still feels that the border is unnecessary.

3. Absolute paths to images: this makes it difficult to share mockups with other people - unless they put in the exact same place on a disk, masters do not automatically load. Perhaps, the logic for encoding file system paths could be tweaked a bit? If the image in the same folder as the mockup, use the relative path?

Thanks!