WordPress reduces image quality to 60% on upload when processing. Your theme templates are designed to use a specific copy of your uploaded image that is scaled or cropped (or both) for the best fit and most efficient load time, so those copies will look different from the original if compression is a problem. For example, in Photogenic, Album posts will display the scaled 1080px image, whereas the click produces the original in a new tab or lightbox. In most cases, the quality change is not noticable, but if your original image is already pre-processed or compressed, the 60% reduction can cause artifacts or visual quality loss.
If you want to prevent WordPress from compressing files or increase how much it compresses files, you can make a change to functions.php in your Theme, or install a plugin to manage the compression level such as WP Smush.it (recommended if you do not have WordPress development experience)
This is considered a modification, and is provided as-is. We expect you understand what you are doing if applying coding changes using these or other WordPress tutorials. Make a backup of your existing file so you can replace it if something goes wrong and you need to start over. Modified files WILL probably be replaced by a theme update at some point, so keep note of your changes for future reference or see our guides below. We cannot assist you with modifying your theme or support problems you encounter as a result!
If you are modifying a theme as part of a client project, the following are really important!How to Modify Your Theme the Right Way
How to Track Update Changes and Maintain Modifications
add_filter( 'jpeg_quality', create_function( '', 'return 80;' ) );
The 80 noted above is the percentage of compression. You can turn this up to 100 if you wish, but then it is doubly important your images are optimized for the web prior to upload to avoid slow load times.
Note that this change will not affect images you have already uploaded. To regenerate all images using the new rules, you will need to download and run the Regenerate Thumbnails plugin.