Hope this solution will work for you.
Take file /lib/Varien/Image/Adapter/Gd2.php and save it to
/app/code/local/Varien/Image/Adapter/Gd2.php
Please find these lines to be at line 147.
// set quality param for JPG file type
if (!is_null($this->quality()) && $this->_fileType == IMAGETYPE_JPEG)
{
$functionParameters
[] = $this->quality();
}
Replace the above line with the following line.
// set quality param for JPG file type
if (!is_null($this->quality()) && $this->_fileType == IMAGETYPE_JPEG)
{
$functionParameters[] = 90;
}
After saving, flush your catalog image cache (from System -> Cache Management in the Magento backend) then refresh your store and see if that sorts out your problem.