Faltering drawing on a 2D image


I’m new to Rhino. I’m trying to make a 2D projection of a building.
First of all I needed to insert the image. For that I used ‘pictureframe’ command and I locked the image. The next step would be to find the vanishing points of this building. When I try to draw a line on the image, it doesn’t let me click where I want to. The cursor jumps from one place to another.
Could it be the reason that the image is not seen as surface but as an image? Is there a command to solve this?
Thanks in advance


Hi Kiiti- most likely you have Grid Snap enabled in the status bar at the bottom of the screen- if the txt for Grid Snap is Bold then it is active - click to disable.