If you're using that in a post, it might just be a quirk of dreamwidth's? I don't run into that problem if I have just one image centered, but once I have multiple in the same centered row, I tend to get a wider space on the left side. Sometimes things format weird on dreamwidth in ways they don't others- even livejournal and insanejournal, despite using the same base code. I've tried a couple ways to counteract it and haven't found a solution yet, sorry D: