title: running a pillow alpha_composite on two files results in a huge huge file
body: <p>I have two files I open via these lines:</p>
  <pre><code>image0 = Image.open(infile0).convert(&quot;RGBA&quot;)
  image1 = Image.open(infile1).convert(&quot;RGBA&quot;).resize(image0.size)
  </code></pre>
  <p>image0 is a 8MB jpg.
  image1 is a tiny 4.3K file that is an overlay to the first one, so it is resized to be the same size as image0.</p>
  <p>After I then run the alpha_composite command:</p>
  <pre><code>result = Image.alpha_composite(image0, image1)
  result.save(outfile)
  </code></pre>
  <p>Where outfile ends up being a huge 23-43MB PNG file.  This seems way to large (I also do this once a minute and create time lapses using FFMPEG which seem to take too long because of the large size of the input files).  I want the file to be as detailed as possible, I would like it to be a tad smaller and more manageable in size.<br />
  I had some success using this command:</p>
  <pre><code>result2 = result.resize((round(result.size[0]*0.20), round(result.size[1]*0.20)))
  </code></pre>
  <p>then doing a result2.save(thumbnail_path), though that file size does seem the quality suffered some, so not sure if I am doing it correctly or if there is a better way in Pillow?</p>