Here's the solution ...
The problem was introduced by new function in 1.5.3 namely getFormatedName() in /classes/ImageType.php which returns $theme_name.'_'.$name_without_theme_name by default if it cannot find the definition under Preferences > Images.
It should return $name_without_theme_name
I think the idea was so that a theme developer could specify the images' sizes using the Preferences > Images.
For example, theme 'mytheme' could specify different size of by adding 'small_mytheme' to the Preferences > Images.
Now, this introduces at least another problem:
If one changes the Theme's name, then one has to change the name of all images under Preferences > Images pertaining to the Theme's name.
I think a better solution is to implement this image size feature on the Preferences > Themes just like the new 'Mail Color" feature.
Therefore, you have 2 options:
- Add image size definitions on the Preferences > Images (e.g. 'small_mytheme')
- Change (or copy it to override folder and modify it) the root of the problem in file /classes/ImageType.php on line 153