It would help to have a bit more information, firstly which module are you using?
This does still sound like a caching issue. You should clear the Joomla cache as well as the module image cache.
You can check if the correct image is being read, by checking the url of the image. The easiest way to do this, if you are using Firefox is to right-click the image, select copy image location, then paste it into a text editor. The small size image will _S at the end of the image file name, the extra small will have _XS at the end.