Layout Not Inflating in Custom Adapter (Using Picasso)

Question

I'm filling in a Gridview with images from the web using the Picasso Library. I'm using a custom adapter.... Within the GetView() method of that adapter, I would like each image to be displayed in a layout.XML file that I created called mov_poster.xml, which simply has an ImageView in it:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">

<ImageView
    android:id="@+id/individual_movie"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />
</LinearLayout>

I'm having a hard time inflating this XML and placing the images within the ImageView. It should be rows of images but every time I launch my app, it's just blank. Here is what my adapter looks like:

public class CustomGridAdapter extends BaseAdapter {
private Context mContext;

private String[] mThumbIds;

public CustomGridAdapter(Context context, String[] items) {
    this.mContext = context;
    this.mThumbIds = items;
}

@Override
public int getCount() {
    return mThumbIds.length;
}

@Override
public Object getItem(int position) {
    return mThumbIds[position];
}

@Override
public long getItemId(int position) {
    return 0;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    if (convertView == null){
        LayoutInflater inflater = (LayoutInflater) mContext.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        convertView = inflater.inflate(R.layout.mov_poster, parent, false);
    }

    ImageView imageView = (ImageView)convertView.findViewById(R.id.individual_movie);


    Picasso.with(mContext).load(My image URL).fit().into(imageView);

    return convertView;

}}