If you are unsure whether or not the existing gallery files are being overwritten by your upload, try deleting the existing files on the server first before uploading the new ones. Then you can be sure that the files on the server are, indeed, the new ones.
If you are unsure whether or not your browser's cache is truly clear, try opening the gallery page in a different browser (IE, Firefox, Safari, Chrome) - one that has not been used to view the gallery before.
Also, you could try changing your method of uploading the gallery files.
For example, if you are currently using a web interface to upload the files, try a FTP client program such as FileZilla which will give you greater control over your uploads.
SimpleViewer Support Team.