Welcome aboard mark_alwill, this will be as individual to your site ad personalities are to people.
Think of a few things:
- Understand that BACKGROUND means that, something behind the FOREGROUND (i.e., your content) that compliments it. It should not dominate or draw attention away from the foreground. Think of a red bucket on the beach, the subject is the bucket, not the sand. Adding other objects to the sand - stones, driftwood, bikini clad women - would subtract from the subject.
- Google around for Accessibility and you will find some important recommendations many site owners choose to ignore in the quest for "cool." A background should not have an irregular pattern that interferes with legibility. For the same reasons, your choice of color - hue value, and chroma - should provide sufficient contrast with the foreground (content!) so that it also does not affect legibility.
Keep the background as a subordinate in your design, let the foreground dominate - which may lead you to "no background at all" being the best solution!
My approach has been to use no more than a "watermark" as background. Very faint but just adding a little texture which a plain colour wouldn't have. One thing I quickly learned was not to work with the original colour photo but to convert the image to greyscale and then adjust the rgb balance to a suitable hue.
All doable with a well known free photo editing package.