Plot onto an image using drawnow

I have the following code, which allows the user to plot points on a graph. What I would like to do is to change the background of the graph plot to show some image of a schematic. How would I go about doing this? I would also like the axes to represent pixel values to the nearest integer instead of floating points. Please help me out with this

function draw_lines
  % Click the left mouse button to define a point
  % Drag the mouse to draw a line to the next point and
  % left click again
  % Right click the mouse to stop drawing
  % 
  clc
  clear

    figure('WindowButtonDownFcn',@wbdcb)
    k=1;
    ah = axes('DrawMode','fast');
    grid on;
    axis ([-1000 1000 -1000 1000])%open the file to save the coordinates into

fileID = fopen('coord.txt','w');
%  hl = line('XData',0,'YData',0,'Marker','.');
%  str=['(',num2str(0),num2str(0),')'];
%   
% drawnow

      function wbdcb(src,evnt)        
           if strcmp(get(src,'SelectionType'),'normal')       
             [x,y,str] = disp_point(ah);           %write the coordinates into the file
             A=[x,y];
             fprintf(fileID,'%6.2f %6.2f\n',A);           hl = line('XData',x,'YData',y,'Marker','.');
             text(x,y,str,'VerticalAlignment','bottom');drawnow
             set(src,'WindowButtonMotionFcn',@wbmcb)        
           elseif strcmp(get(src,'SelectionType'),'alt')
             set(src,'WindowButtonMotionFcn','')
             [x,y,str] = disp_point(ah);           %write the coordinates into the file
             A=[x,y];
             fprintf(fileID,'%6.2f %6.2f\n',A);           text(x,y,str,'VerticalAlignment','bottom');drawnow
             fclose(fileID);
          end
          function wbmcb(src,evnt)
             [xn,yn,str] = disp_point(ah);
             xdat = [x,xn];
             ydat = [y,yn];
             set(hl,'XData',xdat,'YData',ydat);
          end  
       end
       function [x,y,str] = disp_point(ah)
  %          if k==1
  %              x=0;y=0;
  %              k=0; 
  %          else
          cp = get(ah,'CurrentPoint');  
          x = cp(1,1);y = cp(1,2);
  %          end
          str = ['(',num2str(x,'%0.3g'),', ',num2str(y,'%0.3g'),')'];    
       end
    end