From Drupal.org http://drupal.org/node/246774
Came across the same problem, found this issue, added the bits and pieces together and found a solution that works for me. Here it is!
Okay, so just to summarize the last two comments into one fool-proof (I think) solution:
1. Open modules/image/contrib/image_attach/image_attach.module in your favorite editor
2. Copy the entire function theme_image_attach_body and paste it into your template.php
3. Add the following code exactly before the return-statement:
Resulting in the following function (added a little comment as well):
Don't forget to replace "mytheme" with the machine-readable name of your theme.
Note that the caption is wrapped in a div with class "image-attach-caption".
Now the caption may appear at the top of your article, instead of under the image. This is because css that comes with image_attach adds a float:right to the img-tag, instead of the whole parent-div. The solution which worked for me (but might not work entirely satisfactory with multiple images):
Add the following CSS-code to your theme's CSS:
This was done using Drupal 6.16 with Image 6.x-1.0-beta5.