As you can see, I never did receive a reply to my query.
What I did do was reduce the thumbnail size to 100 x 100 and place the code in a <div>
<div align="center">{gallery}guests{/gallery}</div>
You can see the results here
heronshoal.com/guest-pictures.html
I haven't found this forum helpful in the least which makes me suspect that an investment (small as it might be) in any of the paid apps would not be very wise if I needed any sort of support...
HTH