How do I delete Strokes from layers in same file

I do a snipping tool screen shot of a webpage and save it as a png.  Since the web page is actually longer, i do the same with the next portion of the web page in a separate file.  I then open both png files separately and duplicate the second one in the first file ON TOP and move the second one down to where it overlays the first one in the same area (making the second file 50% opacity). After they both line up I make the second one fully visible and a thin STROKE ? line shows at the top of the second layer. I do remember that I created a block in the same color as the background of the first one to erase an ad. That also created a thin stroke line, but I managed to permanently erase that somehow. That would have been a 3rd layer.  I assume that the stroke line in both the 2nd and third layers can be removed the same way, permanently.