Yes, you can do as Andre suggests. However, I think you need to speak to someone at your hosting company with a little more knowledge. As I previously stated, the reson the script cannot write to those files it because of a permissions problem. This is not on the coppermine side, but on a server/filesystem level. What has probably happened is that at some time either you have uploaded files by ftp or your host has restored from a backup or for some reasone permissions have changed. When this occurs and depending on how those files were written apache assigns the ownership of those files to a specific group. Depending on the security settings and the way apache is configured you may find that even setting those files to 777 will not give the script adequate permission to write to those files. You may even find that your ftp package cannot write to those files but hopefully that will not be the case. Either way, it is very simple for someone with a little knowledge at your host to fix. They simply need to change the ownership of all files in the albums directory to something that the script will have permissions to write to. for instance (and this is an example of the command on my machine, it will vary on your hosts system) - chown -R www-data.www-data /cpg/albums
Re-upgrading coppermine will not help in any way with this problem. While Andre proposes a quick fix it is better IMO to learn the correct and simpler way to do this. If you encounter the same problem in the future you will be well armed to rapidly fix it. You may also find you have many sub-directories to go through when deleting all the _thumb files which may take a lot of effort. SImply changing the file permissions/ownership takes seconds.