using pure HTML:
<div style="text-align:center;">[img[...]]</div>
using wikitext inline CSS:
@@display:block;text-align:center;[img[...]]@@
using a CSS rule in a tiddler tagged with $:/tags/Stylesheet:
.center { display:block; text-align:center; }
and then write:
@@.center [img[...]]@@
Note that "display:block" is required for centering to work, otherwise the left/right extents of the containing element fit the content (i.e., match the image dimensions) and there will be no extra whitespace to center the image within.
enjoy,
-e