You should be able to do something like this:
add_action("manage_posts_custom_column", "my_custom_columns");
add_filter("manage_edit-[POSTTYPE]_columns", "my_new_columns");
function my_new_columns($columns)
{
$columns = array(
"image" => "Image"
);
return $columns;
}
function my_custom_columns($column)
{
global $post;
if ("ID" == $column) echo $post->ID;
elseif ("image" == $column) echo 'default_value';
}
Then you should be able to use $tax_term->image
to show it.
This is untested but see this article for more info. Also look into the hooks manage_edit-post_type_columns and manage_posts_custom_column You may also want to edit the code to show an image upload field instead of a text field but it at least gets you started.