Express Border uses Monochrome Threshold to separate shapes from the background, leaving black shapes on a white background. A border is placed around any solid or fully enclosed shapes within the picture. If there are shapes within the shapes, the inner shapes are ignored.
If a picture has a black or very dark border or background, a rectangular border may be created around the edge of the picture, rather than around the desired image. It may not be possible to remove this unwanted border with Monochrome Threshold.
For pictures with a dark border, you could remove the border on the Crop Picture page of the ExpressDesign Assistant, or your could edit the picture to remove the border or change the border color.
For pictures on a dark background, use a picture editor to invert the colors in the picture or change the background color to a light color.
After editing the picture, load it again in the ExpressDesign Assistant.