Issue:
Your thumbnails at your online store and / or category pages have different sizes. Some product images are pixelated.

Solution:

Install (per example) the plugin: ‘Regenerate Thumbnails’ by Viper007Bond

Note: If you don’t want all your images to be regenerated throughout you complete site, then it’s also possible to edit them one by one via the Media Library.

Step 1.
Activate the Regenerate thumbnails plugin.
Step 2.
Go to your Dashboard / Settings / Media
Set the sizes you want for your Thumbnails, medium and large size.
Step 3.
Go to Tools / Regenerate Thumbnails and hit Regenerate all thumbnails.

It may take a while before the regenerating process is finished, and it depends of the amount of images. If you look at your site right now, you’ll notice the adjusted thumbnail sizes (media settings), and they are all regenerated.