Do you mean slow loading time for thumbnails or full size images? ... or both?
For thumbnails, there might be a logical explanation. If for example you have not set permissions or its not creating thumbnails on server, it will will be slow because it has to resize images on the fly for every visitor. However, for full size images, there is no explanation. They are loaded independant from any PHP scripts and the SWF does not contrl in any way how fast they load.