How to detect a picturebox is on a drawn line?

can you provide me some sample code and line is horizontal or vertical?

short of going into machine language, i don't think you can do this.  You can, however, remember your line coordinates in memory and then calculate with the picturebox's Bounds and Location property, whether or not it was placed on the line.
If its a horizontal line and you want to know if the picturebox was placed on top, then check the (Location + Bounds).Bottom is near the line y coordinates and check if the left and right bounds + Location of the picture box fall in between the two x coordinates of the line.