Index: trunk/test/unit/org/openstreetmap/josm/gui/layer/geoimage/ThumbsLoaderTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/gui/layer/geoimage/ThumbsLoaderTest.java	(revision 17867)
+++ trunk/test/unit/org/openstreetmap/josm/gui/layer/geoimage/ThumbsLoaderTest.java	(revision 17870)
@@ -15,4 +15,7 @@
 import java.util.List;
 import java.util.stream.Collectors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 
 /**
@@ -39,4 +42,10 @@
         }
         new ThumbsLoader(imageEntries).run();
+        for (ImageEntry imageEntry : imageEntries) {
+            assertNotNull(imageEntry.getThumbnail());
+            assertEquals(ThumbsLoader.maxSize, Math.max(
+                    imageEntry.getThumbnail().getWidth(null),
+                    imageEntry.getThumbnail().getHeight(null)));
+        }
     }