I have checked that when I circle crop an image element (or callout element with an image) the image is not well seen in retina displays (my mobile is retina display, I dont know in other mobile displays not retina).
I give you an example: http://congruentia.es.mialias.net/blur-circle-crop-test/
They are the same image. I selected the medium size of the image (300x200) in both elements. In left side image I select Crop -> Circle. In right side image I select Crop ->None.
If you see in retina display and compare both images, the upper one is blurred.
To enter in the web:
How can I solve this issue? Is it a bug?