Try to rescan or take a high-quality signature image and convert it into a high-resolution JPEG or PNG format before inserting it into a Word document. You can also check the settings of the Word document before inserting the signature to ensure that the "Compress image" option is not selected, so as to avoid losing image quality.

Black line phenomenon refers to the appearance of a black line or other distortion effect when inserting or saving a handwritten signature image in a Word document, which makes the signature image unable to be displayed or recognized normally, and will affect the aesthetics and readability of the Word document, so corresponding measures need to be taken to avoid or solve it.