diff --git a/Source/Core/Data/DataManager.cs b/Source/Core/Data/DataManager.cs
index c68607e2..c5cf72d0 100644
--- a/Source/Core/Data/DataManager.cs
+++ b/Source/Core/Data/DataManager.cs
@@ -566,7 +566,7 @@ namespace CodeImp.DoomBuilder.Data
 						}
 						
 						// Unload this image?
-						if(!image.IsReferenced && (image.ImageState != ImageLoadState.None))
+						if(!image.IsReferenced && image.AllowUnload && (image.ImageState != ImageLoadState.None))
 						{
 							// Still unreferenced?
 							image.UnloadImage();
@@ -639,7 +639,7 @@ namespace CodeImp.DoomBuilder.Data
 			}
 			
 			// Unload this image?
-			if((img.ImageState == ImageLoadState.Ready) && !img.IsReferenced)
+			if((img.ImageState == ImageLoadState.Ready) && !img.IsReferenced && img.AllowUnload)
 			{
 				// Add for unloading
 				img.ImageState = ImageLoadState.Unloading;
@@ -1087,6 +1087,7 @@ namespace CodeImp.DoomBuilder.Data
 			{
 				ImageData img = new FileImage(Path.GetFileNameWithoutExtension(spritefile).ToLowerInvariant(), spritefile, false);
 				img.LoadImage();
+				img.AllowUnload = false;
 				internalsprites.Add(img.Name, img);
 			}
 			
@@ -1095,6 +1096,7 @@ namespace CodeImp.DoomBuilder.Data
 			{
 				ImageData img = new ResourceImage("CodeImp.DoomBuilder.Resources.Nothing.png");
 				img.LoadImage();
+				img.AllowUnload = false;
 				internalsprites.Add("nothing", img);
 			}
 			
@@ -1102,6 +1104,7 @@ namespace CodeImp.DoomBuilder.Data
 			{
 				ImageData img = new ResourceImage("CodeImp.DoomBuilder.Resources.UnknownThing.png");
 				img.LoadImage();
+				img.AllowUnload = false;
 				internalsprites.Add("unknownthing", img);
 			}
 		}
diff --git a/Source/Core/Data/ImageData.cs b/Source/Core/Data/ImageData.cs
index 94db9aca..a3cb00e6 100644
--- a/Source/Core/Data/ImageData.cs
+++ b/Source/Core/Data/ImageData.cs
@@ -59,6 +59,7 @@ namespace CodeImp.DoomBuilder.Data
 		private volatile ImageLoadState imagestate;
 		private volatile int previewindex;
 		protected volatile bool loadfailed;
+		private volatile bool allowunload;
 		
 		// References
 		private volatile bool usedinmap;
@@ -86,6 +87,7 @@ namespace CodeImp.DoomBuilder.Data
 		public bool IsImageLoaded { get { return (imagestate == ImageLoadState.Ready); } }
 		public bool LoadFailed { get { return loadfailed; } }
 		public bool IsDisposed { get { return isdisposed; } }
+		public bool AllowUnload { get { return allowunload; } set { allowunload = value; } }
 		public ImageLoadState ImageState { get { return imagestate; } internal set { imagestate = value; } }
 		public ImageLoadState PreviewState { get { return previewstate; } internal set { previewstate = value; } }
 		public bool IsReferenced { get { return (references > 0) || usedinmap; } }
@@ -108,6 +110,7 @@ namespace CodeImp.DoomBuilder.Data
 		{
 			// Defaults
 			usecolorcorrection = true;
+			allowunload = true;
 		}
 
 		// Destructor
diff --git a/Source/Core/Windows/ThingEditForm.cs b/Source/Core/Windows/ThingEditForm.cs
index 984300b0..5e805bc0 100644
--- a/Source/Core/Windows/ThingEditForm.cs
+++ b/Source/Core/Windows/ThingEditForm.cs
@@ -185,12 +185,22 @@ namespace CodeImp.DoomBuilder.Windows
 		// Selected type changes
 		private void thingtype_OnTypeChanged(ThingTypeInfo value)
 		{
-			// Update preview image
 			thinginfo = value;
-			if(value != null)
-				General.DisplayZoomedImage(spritetex, General.Map.Data.GetSpriteImage(value.Sprite).GetPreview());
+
+			// Update preview image
+			if(thinginfo.Sprite.ToLowerInvariant().StartsWith(DataManager.INTERNAL_PREFIX) &&
+			   (thinginfo.Sprite.Length > DataManager.INTERNAL_PREFIX.Length))
+			{
+				General.DisplayZoomedImage(spritetex, General.Map.Data.GetSpriteImage(thinginfo.Sprite).GetBitmap());
+			}
+			else if((thinginfo.Sprite.Length <= 8) && (thinginfo.Sprite.Length > 0))
+			{
+				General.DisplayZoomedImage(spritetex, General.Map.Data.GetSpriteImage(thinginfo.Sprite).GetPreview());
+			}
 			else
-				General.DisplayZoomedImage(spritetex, null);
+			{
+				spritetex.BackgroundImage = null;
+			}
 			
 			// Update arguments
 			action_ValueChanges(this, EventArgs.Empty);