Finally fixed the issue where the artist image loading blocks the image loaders queue. This is done by using a custom Executor which is basically a wrapper for two independent ThreadPoolExecutors.

app/src/main/java/com/kabouzeid/gramophone/App.java

@@ -4,6 +4,7 @@ import android.app.Application;
 import android.content.Context;
 
 import com.crashlytics.android.Crashlytics;
+import com.kabouzeid.gramophone.imageloader.PhonographExecutor;
 import com.kabouzeid.gramophone.imageloader.PhonographImageDownloader;
 import com.nostra13.universalimageloader.core.ImageLoader;
 import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;
@@ -38,6 +39,7 @@ public class App extends Application {
 
         ImageLoaderConfiguration config = new ImageLoaderConfiguration.Builder(this)
                 .imageDownloader(new PhonographImageDownloader(this))
+                .taskExecutor(new PhonographExecutor())
                 .memoryCacheSizePercentage(30)
                 .build();
         ImageLoader.getInstance().init(config);

app/src/main/java/com/kabouzeid/gramophone/imageloader/PhonographExecutor.java

@@ -0,0 +1,60 @@
+package com.kabouzeid.gramophone.imageloader;
+
+import android.support.annotation.NonNull;
+
+import com.nostra13.universalimageloader.core.DefaultConfigurationFactory;
+import com.nostra13.universalimageloader.core.ImageLoader;
+import com.nostra13.universalimageloader.core.ImageLoaderConfiguration;
+import com.nostra13.universalimageloader.core.LoadAndDisplayImageTask;
+import com.nostra13.universalimageloader.core.download.ImageDownloader;
+
+import java.io.File;
+import java.util.concurrent.Executor;
+
+/**
+ * A custom {@link Executor} meant for kabouzeid's fork of nostra13's Android Universal Image Loader (https://github.com/kabouzeid/Android-Universal-Image-Loader).
+ * This {@link Executor} separates network and disk loading tasks into different executors so the network image loading doesn't block the disk image loading which is in most cases much faster.
+ * <p/>
+ * Maybe there is a better solution for this with a single (ThreadPool-)Executor, but I'm lacking experience here.
+ *
+ * @author Karim Abou Zeid (kabouzeid)
+ */
+public class PhonographExecutor implements Executor {
+    public static final String TAG = PhonographExecutor.class.getSimpleName();
+
+    private Executor localTaskExecutor;
+    private Executor networkTaskExecutor;
+
+    // The thread pool size here needs further testing. Maybe the 2 additional Threads of the networkTaskExecutor are to much for lower end devices.
+    public PhonographExecutor() {
+        localTaskExecutor = DefaultConfigurationFactory.createExecutor(
+                ImageLoaderConfiguration.Builder.DEFAULT_THREAD_POOL_SIZE,
+                ImageLoaderConfiguration.Builder.DEFAULT_THREAD_PRIORITY,
+                ImageLoaderConfiguration.Builder.DEFAULT_TASK_PROCESSING_TYPE
+        );
+
+        networkTaskExecutor = DefaultConfigurationFactory.createExecutor(
+                2,
+                ImageLoaderConfiguration.Builder.DEFAULT_THREAD_PRIORITY,
+                ImageLoaderConfiguration.Builder.DEFAULT_TASK_PROCESSING_TYPE
+        );
+    }
+
+    @Override
+    public void execute(@NonNull Runnable command) {
+        if (command instanceof LoadAndDisplayImageTask) {
+            String uri = ((LoadAndDisplayImageTask) command).getLoadingUri();
+            switch (ImageDownloader.Scheme.ofUri(uri)) {
+                case HTTP:
+                case HTTPS:
+                    File imageFile = ImageLoader.getInstance().getDiskCache().get(uri);
+                    if (imageFile == null || !imageFile.exists() || imageFile.length() <= 0) {
+                        // the image is not yet in the disk cache
+                        networkTaskExecutor.execute(command);
+                        return;
+                    }
+            }
+        }
+        localTaskExecutor.execute(command);
+    }
+}

app/src/main/java/com/kabouzeid/gramophone/imageloader/PhonographImageDownloader.java

@@ -55,6 +55,7 @@ public class PhonographImageDownloader extends BaseImageDownloader {
         return super.getStream(imageUri, extra);
     }
 
+    @Nullable
     protected InputStream getStreamFromArtist(@NonNull String imageUri, @NonNull Object extra) throws IOException {
         String[] data = imageUri.substring(SCHEME_ARTIST.length()).split("#", 2);
         String artistName = data[1];