How can I change the position of a text overlay?

You can use the gravity parameter (g in URLs) to place your text in different areas of the image. In addition, you can add the x and y parameters for even more accurate positioning.

For example, using the default text placing (center):
http://res.cloudinary.com/demo/image/upload/w_200,h_170,c_fill,g_face/l_text:dark_name/face_center.jpg

And here's a sample of changing the text's location to the south-east corner, with a 5 pixels offset (for both 'x' and 'y'):
http://res.cloudinary.com/demo/image/upload/w_200,h_170,c_fill,g_face/l_text:dark_name,g_south_east,x_5,y_5/face_center.jpg

For more information:
http://cloudinary.com/blog/adding_watermarks_credits_badges_and_text_overlays_to_images