Well, I created some flags for the pre-WW2 German Kriegsmarine:

2 Sail Emblems and 1 Flag.

Just take a look, you will notice the problem: I get a gray border at the edges of the sail emblems:

(I included source material and the .dds files in the ZIP-file)

Well, I guess you already solved the problem and know why it happened.

The bottom, top, left side, or right side of the image cannot touch the borders of the field. Here's what I mean; the yellow represents where it is touching.
http://img131.exs.cx/img131/6797/c1-Skull.jpg