When choosing an image to make an Encaustic painting, I look for 3 important things:
It must be a good image! Don’t think waxing is going to solve a bad image. Your picture must have good “bones” to start with. i.e. leading lines, rule of thirds, etc.
Look for an image that has a lot of white space in which to paint, as there is not a lot to paint when the image is too dark.
You must have a sturdy paper to hold the wax and paint without cracking. Choose an Archival Paper that is 308g/m or better. I find Hahnemuhle Natural or Bight 308gm or Epson Hot Press or Cold Press paper 330 g/m to work the best!