Open a new plan and click on the 'Upload file' button and select your image.
Once your image is uploaded to your Roomle plan, automatically a wall will appear in the right down corner - you can delete it afterwards.
In the middle of your plan you will see a ruler. Scale up the image and use the ruler to set the correct proportions of your plan. In this way, when you draw a wall over the image, it will have the correct measurements.
When you are finished, click on the 'Apply' button. You can now start drawing the walls using the image for reference.
Note that once a room is complete, it will hide the image under it. Thus, you might draw the rooms one by one so that no walls from the image are hidden before you draw them.
When you are finished drawing the walls, click again on the 'Upload File'-button.
The image will come in the front of the plan - you can edit the proportions you set in the beginning and click on 'Apply' to save the changes. Or if your plan is complete, you can delete the image.