@Ashley : I have issues with image editor for a long while now. Not sure how to reproduce it, but it's simple: you have an image, lets say 256x256, and you crop it, sometimes it will not fully crop, but leave 1px of transparent border that is unable to be removed or cropped any further.
For guys that have problems with seames, it might be scaling issue if you're project runs in unappropriate resolution. I always choose halves, quarters and such. For my current project, resolution is set 960x540, which is half of 1080, end even more, originally game was running in 480x270, so assets are only 16x16 and 32x32. At this way selected resolution, I have no problems with any seams, unless webGl effect is being applied to object itself, like floor tiles made out of sprites,something even as simple as blur, then seames appear ( no clue why ). My other settings are: Low quality scaling, point sampling, medium quality downscaling, pixel rounding Off, and webGl On. I use MagiCam plugins to zoom in, to get that original 480x270.