Plotting streamlines on top of a tiff-image

Hey there.

I have made a 2D numeric groundwater model, and I would like to plot results on top of an image in MatLab.

So far I have:

 [h_x,h_y]=gradient(-Head); % Head is a 137x168-matrix
 set(gca,'ydir','reverse'); 
 A= imread('Modelomraade','png'); % Reads the image as a 7376x9000x3-matrix. Bad quality I would like the command below to work
% A= imread('Modelomraade','tiff'); % Reads the image as a 7376x9000x4-matrix, but I'm not able to show this matrix
 figure;imshow(A);title('Simulerede strømningslinjer');
 hold on
 streamslice(h_x,h_y,5); % Draws streamlines with direction arrows.
 hold off

This gives me:

As you can see, I have a problem with the dimensions. The streamlines are in the top left corner of the image, but should be covering most of the picture.

I'm left with two problems:

1. How do I show the image from the tiff-matrix instead of the png-matrix, which has bad resolution?
2. How do I scale the two element, so they're on top of eachother and have the same dimensions?

Please help me!