I have created a circle at the size I want. I now need to place a picture in the exact centre of the circle.  Is there a button on Inkscape that will do this for me or do I have to rely on my eyes?

I am a complete newbie...

Maybe you have seen this?

http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Snapping.html

I use align and distribute, align hor and vertical.

Thank you for those suggestions. The issue seems to lie in the clip art I was trying to use. I had to trust my eyes in the end.