ToMedia.cs

using System;
using System.IO;
using Sitecore.Data.Items;
using Sitecore.Resources.Media;
using Sitecore.SharedSource.DataImporter.Mappings.Fields;
using Sitecore.SharedSource.DataImporter.Providers;

namespace Sitecore.SharedSource.DataImporter.Mappings.Fields
{
    public class ToMedia : ToText
    {
        public ToMedia(Item i)
            : base(i)
        {}

        public override void FillField(BaseDataMap map, ref Item newItem, string importValue)
        {
            var bytes = Convert.FromBase64String(importValue);
            var media = MediaManager.GetMedia(newItem);
            
            // here upload the picture as a .jpg
            media.SetStream(new MemoryStream(bytes, false), "jpg");
            
            // it's needed to make it editable after SetStream becauase it closes the edit context
            if (!newItem.Editing.IsEditing)
            {
                newItem.Editing.BeginEdit();
            }
        }
    }
}