This worked for me:
I = imread('peppers.png'); %# Load a sample image
imshow(I); %# Display it
[r,c,d] = size(I); %# Get the image size
set(gca,'Units','normalized','Position',[0 0 1 1]); %# Modify axes size
set(gcf,'Units','pixels','Position',[200 200 c r]); %# Modify figure size
hold on;
plot(100,100,'r*'); %# Plot something over the image
f = getframe(gcf); %# Capture the current window
imwrite(f.cdata,'image2.jpg'); %# Save the frame data