It might be possible, but it would be dependent a limitation of the Cascading Style Sheets specifications, not of Plogger itself. At the moment, thumbnails are floated left. They could be floated right, if you preferred, but there is no centering property. When I finish typing this, I might take a break from a research project (entirely unrelated to computer science) and look at how one might accomplish your desired effect.
It's complicated by the fact that, if you didn't know, every browser handles CSS differently, and some, like Internet Explorer (5.5, 6.0 and even 7.0 Beta), just get a failing grade altogether.