I'm not expecting a professional photo, although that doesn't hurt. Some of the obvious things to avoid when choosing a photo to upload:

• Blurry photos
  
• Poorly lit photos
  
• Overly filtered photos (I'm looking at you Instagram)
• Inappropriate photos (hey, your pet isn't always an angel)
  
• Excessively cropped photos (I may chose to crop the composition in the final piece, but I can't uncrop a photo or guess what the missing parts of your dog look like)