Twenty Fifteen Image Specs

Quick Specs for images:

  1. The main content width is 660px
  2. The sidebar width is 248px
  3. Featured Images are 825px wide by 510px high
  4. Recommended header size of 954 × 1300 pixels
"Only .jpeg, .gif, and .png files can be used for images in WordPress. All cropped images will be converted to jpegs. Using an image of exactly the right size will produce the best results, and this is certainly recommended. To preserve transparency in an image use a .gif or .png image of exactly the right size."