Using SiteSync to sync over Images from source to target environments corrupts the Images' thumbnails on the target. When the thumbnail is viewed in Chrome, only characters are displayed instead of the image.

In the custom headers section of the web.config on the target environment there is the following custom header applied:
 

<add name="X-Content-Type-Options" value="nosniff" />

MSDN - https://msdn.microsoft.com/en-us/library/gg622941(v=vs.85).aspx

For more information about Bug 229385, see https://feedback.telerik.com/Project/153/Feedback/Details/229385

Until the fix is available, use the workaround below.

There are 2 ways of resolving the problem:

1. Remove the custom header declaration from the target environment's web.config.

2. Implement the following custom solution in the target project's Global.asax:

protected void Application_Start(object sender, EventArgs e)
        {
            Bootstrapper.Initialized += Bootstrapper_Initialized;
        }

        void Bootstrapper_Initialized(object sender, Telerik.Sitefinity.Data.ExecutedEventArgs e)
        {
            if (e.CommandName == "Bootstrapped")
            {
                LibrariesManager.Executing += new EventHandler<Telerik.Sitefinity.Data.ExecutingEventArgs>(LibrariesManager_Executing);               
            }
        }

        private void LibrariesManager_Executing(object sender, Telerik.Sitefinity.Data.ExecutingEventArgs e)
        {
            if (e.CommandName == "CommitTransaction" || e.CommandName == "FlushTransaction")
            {
                var provider = sender as LibrariesDataProvider;
                var dirtyItems = provider.GetDirtyItems();

                if (dirtyItems.Count != 0)
                {
                    foreach (var item in dirtyItems)
                    {
                        //Can be New, Updated, or Deleted
                        SecurityConstants.TransactionActionType itemStatus = provider.GetDirtyItemStatus(item);

                        var tmb = item as Thumbnail;
                        if (tmb != null)
                        {
                            if (itemStatus == SecurityConstants.TransactionActionType.New
                                || itemStatus == SecurityConstants.TransactionActionType.Updated)
                            {
                                if (tmb.MimeType == null)
                                {
                                    var parentImgMimeType = tmb.Parent.MimeType; //we get the MimeType of the Parent (which is the actual image)
                                    tmb.MimeType = parentImgMimeType; 

                                }                             
                            }
                        }
                    }
                }
            }
        }

After compiling the project and running a sync of an Image, the Thumbnail will be displayed correctly in Chrome.

To fix the already Synced images' thumbnails run the following SQL query against your database:
 

update sf_media_thumbnails
set mime_type = sf_media_content.mime_type
from sf_media_thumbnails 
join sf_media_content 
on sf_media_thumbnails.content_id = sf_media_content.content_id
where sf_media_thumbnails.mime_type is NULL