diff --git a/src/main/java/de/thm/arsnova/ImageUtils.java b/src/main/java/de/thm/arsnova/ImageUtils.java
index 19cb75c40c3985b2a564e5cc87bbad8ce9e88e55..9983b4951a353da531da6a13f6d38ffb10f85aae 100644
--- a/src/main/java/de/thm/arsnova/ImageUtils.java
+++ b/src/main/java/de/thm/arsnova/ImageUtils.java
@@ -134,6 +134,9 @@ public class ImageUtils {
 			else {
 				final int extensionStartIndex = IMAGE_PREFIX_START.length();
 				final int extensionEndIndex = maybeImage.indexOf(IMAGE_PREFIX_MIDDLE);
+				if (extensionEndIndex < 0) {
+					return null;
+				}
 
 				final String imageWithoutPrefix = maybeImage.substring(extensionEndIndex);
 
diff --git a/src/test/java/de/thm/arsnova/ImageUtilsTest.java b/src/test/java/de/thm/arsnova/ImageUtilsTest.java
new file mode 100644
index 0000000000000000000000000000000000000000..0a8237721da37bcddcfcbbf383237eda50e54253
--- /dev/null
+++ b/src/test/java/de/thm/arsnova/ImageUtilsTest.java
@@ -0,0 +1,63 @@
+package de.thm.arsnova;
+
+import static de.thm.arsnova.ImageUtils.isBase64EncodedImage;
+import static de.thm.arsnova.ImageUtils.extractImageInfo;
+import static de.thm.arsnova.ImageUtils.IMAGE_PREFIX_START;
+import static de.thm.arsnova.ImageUtils.IMAGE_PREFIX_MIDDLE;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertEquals;
+import org.junit.Test;
+
+public class ImageUtilsTest {
+
+  @Test
+  public void testNullIsNoValidBase64String() {
+    assertFalse("\"null\" is no valid Base64 String.", isBase64EncodedImage(null));
+  }
+
+  @Test
+  public void testEmptyStringIsNoValidBase64String() {
+    assertFalse("The empty String is no valid Base64 String.", isBase64EncodedImage(""));
+  }
+
+  @Test
+  public void testWrongStringIsNoValidBase64String() {
+    final String[] fakeStrings = new String[] {
+      "data:picture/png;base64,IMAGE-DATA",
+      "data:image/png;base63,IMAGE-DATA"
+    };
+
+    for (String fakeString : fakeStrings) {
+      assertFalse(
+        String.format("The String %s is not a valid Base64 String.", fakeString),
+        isBase64EncodedImage(fakeString)
+      );
+    }
+  }
+
+  @Test
+  public void testValidBase64String() {
+    final String imageString = String.format("%spng%sIMAGE-DATA", IMAGE_PREFIX_START, IMAGE_PREFIX_MIDDLE);
+    assertTrue(isBase64EncodedImage(imageString));
+  }
+
+  @Test
+  public void testImageInfoExtraction() {
+    final String extension = "png";
+    final String imageData = "IMAGE-DATA";
+    final String imageString = String.format("%s%s%s%s", IMAGE_PREFIX_START,
+      extension, IMAGE_PREFIX_MIDDLE, imageData);
+
+    final String[] imageInfo = extractImageInfo(imageString);
+    assertNotNull(imageInfo);
+
+    assertEquals("Extracted information doesn't match its specification.", 2, imageInfo.length);
+
+    assertEquals("Extracted extension is invalid.", extension, imageInfo[0]);
+    assertEquals("Extracted Base64-Image String is invalid.", imageData, imageInfo[1]);
+  }
+
+}