I have the same problem. Untill recently I had no borders (or 2 px) around my pictures, but I made a new album and even though the margin says 1px or 2 or even zero I still get a border. Very frustrating. Any suggestions. All the settings as far as I can see are the same as with my other albums.
That setting is to change the size of the border on the large pictures, not on the thumbnails.
The border on the thumbnails is used to make the thumbnails fit evenly in the width you've chosen for the theme image. To get rid of the border on the thumbnails, you have to set the minimum padding to zero (see ss003814), then experiment with the theme image width and thumbnail bounds to get the thumbnails to fit evenly, without a border. For example, leaving the theme image width at 900px, thumbnail image bounds of 222x150 seems to work (see ss003815).
Thanks. I suppose I have an older version of Turtle (4.0.4) and have a different layout of that tab. However I do wonder why I never had this problem with all my previous maps, apart from the 1 I made in October 2017, when the problem occurred for the first time. Could it be the fact that I have this older version of Turtle?