Clear method simply draws the background of the graphics to the provided color code.
As per Varocarbas comment above question,
"The explanation you are posting sounds pretty descriptive: it sets the background of the graphic (future image) to the given color. From what you are saying, your problem is not provoked because this code "draws lines", but because it makes the whole background white and the size of bmpReturn does not occupy all this background (and thus the white lines you see are not more than parts of the background). If you don't want this effect, just don't use this line of code; and, in general, you shouldn't use code whose exact functionality is not clear to you"