You need to have some sampling buffer around your sprites.
Here is the same PG but with a properly extended sprite set.
Notice how the artifact no longer is there.
I think the setting in texture packer was set to 6 pixels of extrude. (To accommodate mipmapping if you are not using mipmapping you can probably just do 1 pixel).
this will also make some artifacts that you have to clean up by hand (see on the castles and houses in this example) but that is usually really easy.
This is what it looks like without:
Also when you make the sprites take up about the same screen size they render at the same quality. Can you tell which one of these is from the 3d scene and which is from the flat image?