P.S. In my HTML example, the following code:
suggests that the table (and the things within it) should be centered between the left and right margin of the blog. (Whether it works will depend partly on the template, I think, and partly on the generation of WordPress software being used at the time the blog is viewed. No guarantees.)
Also, the following code:
adjust the position of the content within the table area itself. So the first chunk of content (in my case, the image) will be centered within the centered table (which is also centered -- we hope -- between the left and right margins of the blog). And the second chunk (in my case, the "caption" text) is left-justified within the centered table area.
(The width of the table area depends on the width of the content you put in. This is a lot easier to try than to explain! Using the "Preview" button in WordPress should show you roughly how the table will appear once you publish... though it unfortunately won't show how the table will appear in the emailed version. The email version has different formatting.... partly because the email doesn't have the same width, and margin settings, as the Web version. That makes precise formatting a real problem.. and I hope that this help area has long descriptions of how to work with the WordPress emailed blogs. My client is into print publishing -- books, that is -- and has real heartburn every time she sees our careful formatting altered by the email system. My table is one thing we're going to try to use to override the email mess.)
If you'd like more examples, searching the web for HTML table td align should give lots of examples of table cell alignment.