Hi Giulia,

Kindly find my response for your inquiries below:

our blog editors complain that sometime the cover images are in bad quality and truncated even if the file is in good quality.

-> May I know which page that are you referring this to? is it in the listing or entry view? Actually you can configure the quality of cover image that has been uploaded in the setting.
http://take.ms/pinCL http://take.ms/kSd0V

could you specify what it the right size of images they have to upload?

-> Actually it is depend on the original image resolution and size itself and how big you want it to be display. If the image is already low in quality and small size and you want it to be display large in the site it will display blurry
The image size quality that would be suitable to display nicely more than 600x640px (medium) or large image (1280x1280)

ok thank you
I have changed the settings as you suggested
I let you know if there is any problem

Sure Giulia