You should have two of them - unless you haven't taken the second one yet, of course.
You got the first line from the floor on level 3. That created the upper gap initially (walkthrough page 6). The second line is the one standing vertically left of the yellow box. It is described on page 13 how to get it.
If you haven't got the second line already and used the other one to close the lower gap, then pick it up again, because you will need it to cross the upper gap to get back to the left part of the painting.