I had been having the same problem… I tried this and it seems to be working:
1) Ensure that your image is 310x150px
2) Set up your default image and thumbnail sizes: Click Settings – Media and set your thumbnail size to 310px wide by 150px tall. I also unchecked “Crop thumbnail to exact dimensions,” although I’m not sure this does anything.
3) Set your medium size images to 590px wide. Do not set a height.
4) Set your large size images to 950px wide. Do not set a height.
Now give it a try… Hope that works!