Trouble with Threads blocking execution

// This method is called to create a PictureBox's Image
private Image GetThumbnail(
string uri, // the filepath to the image
PictureBox pictureBox, // the PictureBox control - we need to know this so the worker thread can replace the right PictureBox's Image
int width, // the thumbnail width
int heigh // and the height
)
{
Image thumbnail = GetIcon(uri); // GetIcon returns an Image based on the filetype of the specified URI
ThumbnailThread thumbnailThread = new ThumbnailThread(uri, pictureBox, width, height, ThumbnailCallback); // See below for more on the ThumbnailThread class
Thread thread = new Thread(thumbnailThread.ThreadProc);
thread.Start();
return thumbnail; // we return the temporary thumbnail and the thread will update the PictureBox with the full image preview later
}

// This callback actually swaps the PictureBox's Image
private void ThumbnailCallback(Image image, PictureBox pictureBox)
{
pictureBox.Image = image;
}
and the ThumbnailThread class is thusly defined:
public class ThumbnailThread
{
private string _uri; // the filepath to the image
private PictureBox _pictureBox; // the PictureBox control
private int _width; // the width of the thumbnail
private int _height; // and the height
private ThumbnailThreadCallback _callback; // the callback method

public ThumbnailThread(string uri, PictureBox pictureBox, int width, int height, ThumbnailThreadCallback callback)
{
_uri = uri;
_pictureBox = pictureBox;
_width = width;
_height = height;
_callback = callback;
}

public void ThreadProc()
{
Image thumbnail = Image.FromFile(_uri); // get the whole image
thumbnail = MyClass.FixedSize(thumbnail, _width, _height); // FixedSize is a static method of MyClass that handles fixed proportion resizing
_callback(thumbnail, _pictureBox); // callback to the main thread
}
}
and the ThumbnailThreadCallback delegate looks like this:
public delegate void ThumbnailThreadCallback(Image image, PictureBox pictureBox);