change episode images from png to jpeg

[mattstratton]

Similar to #233

Few steps needed here:

• Convert all existing episode images to JPEG (leaving the PNG files in place)
• Update the code to look for .jpg instead of .png when pulling up an episode image
• Update the README to tell people to make a JPEG instead of a PNG
• Update the README to specify a larger size for the square episode image for use in cover art
• Check the use of the square cover art in the homepage code to constrain the image size so it doesn't blow up

[mattstratton]

With the new theme, this will work going forward using whichever type of image you want.