I believe ImageData.xml and gallery.xml are the same file, but for different versions of simpleviewer.
One of the fundamental aspects of XML is that each file has a "root element." That's the first part of the page that is surrounded by < and >. In the case of simpleviewer, that's "simpleviewerGallery". Everything after that name and before the > is an "attribute." These are options that you can set to give information and options to simpleviewer.
The options you want to set are "imagePath" and "thumbsPath." In the example below, I've set mine to be a subdirectory. I'm not sure if you know how URLs work, but if the directory you've got your images and thumbs in is in the same diretory as the XML file, you only need the name. For instance, if you want to put your images in /somepath/gallery_images/, and the XML is in /somepath/, your imagePath would be imagePath="gallery_images/". However, if your XML file was elsewhere, you can use the site-root relative path, and that would be imagePath="/somepath/gallery_images/".
Here's my root element for one of my galleries:
<simpleviewerGallery maxImageWidth="450" maxImageHeight="450" textColor="0x585858" frameColor="0x01adff" frameWidth="1" stagePadding="15" thumbnailColumns="2" thumbnailRows="3" navPosition="left" title="Animals" enableRightClickOpen="true" imagePath="/images/animals/medium/" thumbPath="/images/animals/small/">
I hope I've made this clearer it can be daunting if you've not dealt with this type of thing before. You can also check out my pages at my home page, simon-photography, then dot, then com. My account is too new here to post the link :)