Even though your game isn't pixel art, all raster images are made up of pixels. If you have resized the image in an editor, chances are the editor created a slight color change along the edge. It's called the halo effect.
C2's built in editor definitely does it, and Photoshop does it unless you change the settings to "Nearest Neighbor" when resizing, but that's only really good for pixel art.
It might be a good idea to zoom in on the background image and look for a variation in color along the edge. Even the slightest color variation will create a seam. If you can't tell just by looking at it, try to paint the 1 pixel edge along the outside of the image to see if it breaks the seam.
If you work larger than what you end up using in game, and the art comes to the edge of the canvas, this will always be an issue when resizing. Converting from vector to raster also causes this issue.
Hope that helps.