Thanks to everyone for your replies and sorry I have been a bit slow in replying but life has been dragging me off in another direction and preventing me from getting on with my photo projects'
Having read my question again I can see I could have listed it a bit better to make it more understandable. I was playing around with the layers as I thought that was where it would be done. What I want to do is have one picture and then be able to put a faint image over the top. I suppose an example might be a watermark on a banknote, it is there but very faint and visible when held to the light. I want to do that kind of thing but not quite so faint. The image needs to be transparent so that the top image does not block any of the first photo.
Hopefully I have explained it better now and I hope to try out the suggestions this week and I will post how I get on
Edward