since I am two days behind in updating our site...
Just for a short-term workaround can you upload (or delete) images with ftp and then rebuild the gallery?
Meanwhile, lets try to get to the bottom of the real problem. This could be a clue:
load: class javaatwork.myuploader.UploadApplet.class not found.
The UploadApplet.class file should be inside one of the java archive (.jar) files on your server. I'm not a java expert but afaik .jar files are a bit like .zip files that hold the java code. I think the missing class should be inside svmanager/uploader/myuploader-source-signed-1.13.jar. So why can't it find the class? Could it be missing, corrupted, read-protected?
Try uploading a new copy of this file to your server from the zip file you downloaded from SimpleViewer.net when you upgraded. Make sure it uploads as a binary file, not text. Check that the permissions make it readable. It doesn't need to be writeable.
It's also probably worth uploading a clean copy of the file that creates the html to load the java. That one is svmanager/classes/uploadpage.php and it should upload as a text file.
SimpleViewer Support Team.