think your problem is two fold.
First I'd create a new div called - gallery or what ever you want. Give it a fixed width ( of your choice) and then make the L and R margins auto.
Place the div on your page and then place your php code inside that.
This will ensure it's centred.
Second - as I discovered Power gallery is funny about sizes. Your images are high quality so I assume they are rather large.
If that is the case then I'd resize them to say 800* 600/.
Lastly you will need to make some alterations to your gallery code to ensure that you photos are not cropped automatically my Power Gallery. BY default the dissolve show is 782px wide I suspect that the area allowed on yoru page is not 782 hence the allignment issue.
It's not that I'm clever - I had the same issue and support sent me this link