Welcome Avijit!
Concerning the vertical line, have a look at the tool options tool bar above the drawing area, when the
Vertical Line tool is active. There you can define the what the click position is, start/middle/end of the line and also the length of the line. In your case use end for a vertical line downwards.
You can find examples in our manual at
https://librecad.readthedocs.io/en/latest/ref/toolbars.html#tool-options.
Sadly the vertical line options are not listed there yet.
Concerning the image export, to be honest, I can't say how the export looks like currently. Maybe another user can give a valuable response. I need this very rarely and when I use screenshots.
Armin
investing less than half an hour into Search function can save hours or days of waiting for a solution