diff --git a/cmd/fastgallery/main.go b/cmd/fastgallery/main.go
index eb1f277..8b76f5e 100644
--- a/cmd/fastgallery/main.go
+++ b/cmd/fastgallery/main.go
@@ -306,6 +306,8 @@ func stripExtension(filename string) string {
 	return filename[0 : len(filename)-len(extension)]
 }
 
+// reservedDirectory takes a path and checks whether it's a reserved name,
+// i.e. one of the internal directories used by fastgallery
 func reservedDirectory(path string, config configuration) bool {
 	if path == config.files.thumbnailDir {
 		return true
@@ -325,7 +327,7 @@ func reservedDirectory(path string, config configuration) bool {
 // hasDirectoryChanged checks whether the gallery directory has changed and thus
 // the HTML file needs to be updated. Could be due to:
 // At least one non-existent source file or directory (will be created in gallery)
-// We're doing a cleanup, and at least one non-existent gallery file or directory (will be removed from gallery HTML)
+// We're doing a cleanup, and at least one non-existent gallery file or directory (will be removed from gallery)
 func hasDirectoryChanged(source directory, gallery directory, cleanUp bool) bool {
 	for _, sourceFile := range source.files {
 		if !sourceFile.exists {
@@ -372,7 +374,9 @@ func compareDirectoryTrees(source *directory, gallery *directory, config configu
 		var thumbnailFile, fullsizeFile, originalFile *file
 
 		// Go through all subdirectories, and check the ones that match
-		// the thumbnail, full-size or original subdirectories
+		// the thumbnail, full-size or original subdirectories.
+		// Simultaneously, mark any gallery files which exist in source,
+		// so any clean-up doesn't inadvertently delete them.
 		for h, subDir := range gallery.subdirectories {
 			if subDir.name == config.files.thumbnailDir {
 				for i, outputFile := range gallery.subdirectories[h].files {
diff --git a/cmd/fastgallery/main_test.go b/cmd/fastgallery/main_test.go
index 0723579..b48f0b3 100644
--- a/cmd/fastgallery/main_test.go
+++ b/cmd/fastgallery/main_test.go
@@ -3,6 +3,7 @@ package main
 import (
 	"os"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -312,6 +313,12 @@ func TestCreateDirectoryTree(t *testing.T) {
 	defer emptyFile5.Close()
 	defer os.RemoveAll(tempDir + "/gallery/" + myConfig.files.thumbnailDir + "/file.jpg")
 
+	// Ensure thumbnail file is newer than source file
+	err = os.Chtimes(tempDir+"/gallery/"+myConfig.files.thumbnailDir+"/file.jpg", time.Now(), time.Now())
+	if err != nil {
+		t.Error("couldn't change mtime/atime")
+	}
+
 	emptyFile6, err := os.Create(tempDir + "/gallery/" + myConfig.files.fullsizeDir + "/file.jpg")
 	if err != nil {
 		t.Error("couldn't create original gallery file")
@@ -323,6 +330,8 @@ func TestCreateDirectoryTree(t *testing.T) {
 	gallery := createDirectoryTree(tempDir+"/gallery", "")
 
 	compareDirectoryTrees(&source, &gallery, myConfig)
+	t.Log(source)
+	t.Log(gallery)
 
 	changes := countChanges(source)