If you need an image with transparent background, you can create it in other packages, for example GIMP
In your script, you first need to read the image with the transparent background together with the information about the alpha channel:
[img_data ~ img_alpha] = imread(img_fname); img_data(:,:,4) = img_alpha(:,:);
When opening the psychtoolbox window, enable alpha blending
Screen('BlendFunction', theWindow, GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
Create the texture for the image
img_tex = Screen('MakeTexture', win, double(img_data));
In the main loop of movie playback, overlay the image texture over every movie texture
Screen('DrawTexture', win, movietexture);
Screen('DrawTexture', win, img_tex);
