Question: Using QGIS2.4.0 I just added 2 maps to an existing project. Both are scanned images of pre-1800 plans. One is a jpg file, I georef'd and added to QGIS no problem and plan displays in the project.
I carried out exactly the same process with the other plan, a tif file, and the process completed but the plan just displays as a black rectangle, can anyone advise a very novice novice what might have gone wrong please.

Could it be because the jpg was a colour image, but the tif is a greyscale? I've had problems with greyscale images before.

Answer: 
It might be necessary to add an additional NODATA value for transparency. 

Activate the layer, click on the Info icon, and pick the value from a part of the picture where you know there is no data (like the triangles between the original image content and the rectangle you got after georeferencing).

Then stretch the greyscale again between Min and Max.
I managed to get 2 greyscale TIF files to georef by first opening them with a graphics editer and converting the images to 24bit colour.