You should be able to right-click the outer image, choose Format Picture >
Line Color, and click No Line. (There's also a Picture Border button on the
Picture Tools ribbon that's visible when the picture is selected, but I
understand your dislike of the ribbons.)
If that doesn't remove the line, or if the line color is already set to No
Line, then I'll guess that either (a) you have the Picture Placeholders
option checked (see your other thread) or (b) the line is actually part of
the jpeg file.