There does appear to be a problem with your screen shot. Have you tried saving and opening in another app to see if the line is still there? We'd need to see a full res screen shot of your workspace showing layers and paths panels to have better chance of telling you what is going on.
Lines often show between layers, that are just screen artifacts. If you flatten or create a merged copy of the layers, most often, they will not be there.
My $0.02. Judge in 100% view.
Excellent point Bojan. It usually does come down to that.
If you view at 100% or higher magnification and there is still a line, it will most likely be due to anti-aliasing on the layer edge. If this is the case, it can be dealt with fairly easily, however, I'm guessing it disappears at different zoom levels.
I'm really new at this and I don't know how to flatten or what anti-aliasing is. I have tried "snap to" all and the images will snap to everything but each other. I have merged the images and the line is still there. I need the final product to be of at least 300 dpi.
Thank you. I turned off antialiasing in windows and then merged my images and the line went away.