Its a little bit complicated. I have installed 3 plugins for doing this one: ImageManager, rc:custom_field_gui and Get Custom Field Values... u can found them in the plugin codex section.
Whit the first one... ImageManager ... i'm uploading the image from my computer in an /images/ directory and auto creating the thumbnails in /images/thumb/ directory.
After that i have installd rc:custom_field_gui and addet a simple custom field to all the posts:
type = textfield
Them the 3'rd plugin, Get Custom Field Values, addet the code to the loop of my pages (index.php and single.php)
For the thumb in index.php i have add:
<?php echo c2c_get_custom('Images', '<img alt="images" class="images" src="path/to/my/direcoty/images/thumb/', '.jpg" />', ''); ?>
And for the original format image in single.php:
<?php echo c2c_get_custom('Images', '<img alt="images" class="images" src="path/to/my/direcoty/images/', '.jpg" />', ''); ?>