PHP help with IF statement (connected to Custom Field)

Hello,

I’ve installed wordpress and have been tinkering with my site for a week or so. I am very impressed. I will migrate my sites from drupal.

I have set my site set up to display a different header image depending on the page, which works fine, but really I would like it to be set up like this (don’t know how to code it):

IF custom field=null (where the custom field is called ‘isbn’)
IMG SRC=default.jpg
elseif (display this at all other times)
IMG SRC=otherimage.jpg

This can’t be too hard can it?

any help is greatly appreciated!

Ian