If you want to create a new Line (no difference between Pipe Lines and Signal Lines) you just need to create a new class.
After that you can click on the “Edit Line…” button to modify the Line properties.
Under Symbol Properties you can define whether or not you want a Flow Arrow be placed automatically when drawing this Line.
Gap priority allows you to set a value between 1 and 9. 1 being the highest priority and 9 the lowest. This influences the way Lines are broken when crossing. The line with the higher priority won’t be broken.
When you add a Line to the Tool Palette the Line will be shown as a simple black line. Make a Right-click on it and select “Properties…”.
In the next Dialog you make a Right-Click on the image and select “Specify image…”. This allows you select an image file. The resolution of the image should be 32x32.