• Maximum image size - 800 x 800 px 
  
  
• Maximum attachment size - 1MB (2MB for Enterprise customers)
  

Tips:

• Always use alternative text when using images - make sure it explains what the image is. If the alt text is too long it won't render.
  
• Make your image files as small as possible, so they render quickly and don't set off any spam flags. The smaller your image size, the better. You can use sites like tinypng.com and www.jpegmini.com to quickly reduce your file sizes.