Hmm. According to ยง23.6. in the manual,
display (figure name)
"causes the figure to be displayed in the centre of the screen visible to the player."
But when I try it, it doesn't center the image--it just puts it on the left side. Is that what it does for you? Maybe it's an Inform bug.
EDIT: Ok, from what Draconis said here, you have to include an extension in order to center the image. But the built-in extension that's supposed to do this needs updating. Here's the bug report.
If you don't want to wait for the bug to be fixed, there might be a way to update the extension yourself, and this answer might be of help.
I don't know how one goes about modifying a built-in extension, though.