Hyperlink tag inside paragraph and text-align:center

Hyperlink tag inside paragraph and text-align:center

<div style="width:400px">
<p style="text-align:center">
<a>
<img src"..." />
</a>
Some text..
</p>
</div>
So the goal here is to center the image inside the div. Text-align:center
works on the images that are not inside the hyperlink tag, but once they
are, it doesn't affect them, just the text.
I don't want to go with display:block and margin-left/right auto, simply
because we'll need to align some pictures to the left and right using
float.
Any ideas why the hyperlink prevents text-align to work?