Double check permissions on the imagegallery upload directory and its files. ie apache must be able to read and write to that directory and those files too.
Also clear the contents of templates_c through the System Admin option on the Admin (Click) menu.
Id guess its permissions problem. The files in the upload directories would have been owned by apache:apache or www-data:www-data depending on your host. If they have got moved, the user/group might have changed. a chown and a chmod in those upload directories would probably solve this.