Renaming an image when its uploaded to a property with Alias "umbracoFile", but before its saved. The name of the image will be changed to the name of the folder it resides in. This is using the Saving event in the MediaService in Umbraco.

MediaEventHandler.cs

using Umbraco.Core;
using Umbraco.Core.Events;
using Umbraco.Core.IO;
using Umbraco.Core.Services;

namespace Startup
{
    public class MediaEventHandler : ApplicationEventHandler
    {
        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
        {
            MediaService.Saving += MediaService_Saving;
        }

        void MediaService_Saving(IMediaService sender, SaveEventArgs<Umbraco.Core.Models.IMedia> e)
        {
            var fs = FileSystemProviderManager.Current.GetFileSystemProvider<MediaFileSystem>();
            foreach (var media in e.SavedEntities)
            {
                //Assuming Media item only has one umbracoFile property to handle
                var umbracoFileProperty = media.Properties["umbracoFile"];
                //Path will have this format /media/1234/koala.jpg
                var imagePath = umbracoFileProperty.Value.ToString();
                var fullPath = fs.GetFullPath(imagePath);
                var relativePath = fs.GetRelativePath(fullPath);

                //Update the path for each thumbnail
                var thumbnails = fs.GetThumbnails(relativePath);
                thumbnails.ForEach(thumbnail => SetNewImagePath(thumbnail, true, fs));
                
                //Update the path for the 'main' image
                var newImagePath = SetNewImagePath(relativePath, false, fs);
                
                //Update property with new image path
                umbracoFileProperty.Value = newImagePath;

                //Remove old image and thumbnails
                fs.DeleteFile(relativePath, true);
            }
        }

        private string SetNewImagePath(string imagePath, bool isThumbnail, MediaFileSystem fs)
        {
            var file = fs.GetFileName(imagePath);
            //Extract name of file
            var fileNameNoExt = file.Substring(0, file.LastIndexOf('.'));
            if (isThumbnail)
                fileNameNoExt = fileNameNoExt.Replace("_thumb", "").Replace("_big-thumb", "");
            //Extract name of directory
            var folderName = imagePath.Substring(0, imagePath.IndexOf('\\'));
            //Assemble new path
            var newImagePath = imagePath.Replace(fileNameNoExt, folderName);
            //Move file to new path, and override any existing image
            using (var fileStream = fs.OpenFile(imagePath))
            {
                fs.AddFile(newImagePath, fileStream, true);
            }
            //Return the full path as that is what we want to store
            return fs.GetFullPath(newImagePath);
        }
    }
}