Actually I just thought about this again. It's not so hard.
Open includes/db_field_definitions.php and read the comment block. You'll need to add additional fields to your images table. So if you add a field called "has_flash", you can then put something like this in your details.html
{if has_flash}<img src="{template_url}/images/flash.gif>{endif has_flash}
You also need to add these fields to the member upload form. Again, read the comments inside includes/db_field_definitions.php