Allows setting the default JPEG quality for resized images
You can either delete and re-upload your images, or use a plugin like this one: https://wordpress.org/extend/plugins/regenerate-thumbnails/
At very low quality levels and on slow servers, timeouts can sometimes occur during the creation of thumbnails. This appears to be a problem with the PHP image libraries. To cause these timeouts to occur, one has to set the JPEG quality to a comically low number, one which would never normally be used on an actual site, but the option has been provided.
JPEG image qualities higher than 95 will not normally be perceptible to viewers. Those higher qualities cause the image size to ramp up very quickly, with very little image quality improvement in return. JPEG quality settings over 95 may be useful for archival purposes but are not appropriate for thumbnails. However, the option has been provided for people who want it.
The JPEG quality cannot be set below zero or above one hundred and will be rounded to a whole number.
The default JPEG quality used by WordPress is 90.