WordPress users occasionally notice that the images they upload and insert into their posts appear duller than the originals. That’s because WP isn’t as good as an image editing application when it comes to resizing: images resized by WP lose some color and sharpness.
I first came across the issue in this forum thread (check the example I link to). At the time I wasn’t aware that Richard (a.k.a. thesacredpath), an expert forum volunteer who is a lot more experienced than me, had already written a post on the issue. As that post says, the solution used to be: “pre-size and resample your images with a dedicated image editing program before you upload them”.
Then, one day in mid-August last year, users suddenly began to complain that their GIFs lost their animation or transparency. I knew that resized GIFs don’t work, so I thought that deleting the dimension declarations from the code might correct the problem. True, GIFs with no dimension declarations sprang back to life. And I found that this had an effect on all image files. In other words: after the latest back-end changes, images uploaded and inserted into your posts will lose in quality if their code includes width and height, even when they are inserted in real size.
So, for best image quality, make copies of your images, resize them (using an image editing application) to a width not exceeding the main-column width of the theme you’re using (see here) and a resolution of 72 (so as not to waste storage space), upload and insert those copies, then delete the width and height commands from the image code (in the html editor).
Notice the differences in the following example:
(Image resized by WP)
(Same image, resized prior to uploading)
(Same image, resized prior to uploading, width and height commands deleted after uploading)