You are probably missing ImageMagick (PHP extension). Install it and the JPG artifacts will be gone.
Related Posts:
- Dynamically determine the width and height of a image file to be cropped & uploaded AFTER form submission given fixed aspect ratio
- get the post thumbnail returns the full size
- No Thumbnails Generated
- Is there a hook which fires after all thumbnails are generated?
- Settings in ‘Media > Settings’ is ignored when inserting images
- Convert uploaded PNG to JPEG automatically
- Regenerate thumbnails unexpected resolution (96 x 96)
- add_image_size() and uploading the exact size leaves 1×1 image in uploads
- How to stop thumbnail generation from some images and different size thumbnail generate
- Can’t generate thumbnail images
- Original image not saving?
- How to Make Thumbnail of Post Stay Animated
- adding image to post thumbnail from another server
- WordPress Multisite: Images moved to /sites/1?
- Redimension thumbnail crop without creating new images
- WordPress pointing animated GIF images with wrong file name upon upload (GIFs animation not working)
- How to generate thumbnails when needed only?
- How to Require a Minimum Image Dimension for Uploading?
- How to disable WordPress from creating thumbnails?
- How does WP media uploader create the 3 different sized images, and how can I duplicate it
- set_post_thumbnail_size vs add_image_size
- How to disable generation of default image sizes for some custom post types?
- Reject upload of wrong-sized images using the Media Uploader
- Upload post thumbnail from the front end
- Download an image from a webpage to the default uploads folder
- the_post_thumbnail responsive srcset not populating with custom image size
- New image size version only for the Featured Image file
- Create image formats with different qualities when uploading
- Which filters or actions to use after a media upload and delete?
- PNG with transparent background turns black when uploaded and resized
- Differentiate Featured Image from Post Images upon Upload
- get_the_post_thumbnail_url with an unregistered size
- Modify featured image path to Amazon S3
- Cropped featured image replaces original image in gallery
- There’s a way to scale media (images) at 50%?
- How to prevent upload of a multiple sizes of images
- How can I prevent uploading bmp image?
- Impose a Maximum Limit on Image height and width upload size
- Force wordpress to see uploads folder media
- add_image_size creating lots of images (of various sizes)
- How can I add the “Use as featured image” to a custom metabox?
- Can WordPress crop only the post thumbnails?
- resize images not crop
- No srcset for hard-cropped thumbnails
- Autogenerated Thumbnail compression depending on size
- How to upload imagick resource to media in wordpress
- Insert Featured image from Feed
- Upload image from a URL and assign it as a Featured Image to a Custom Post Type?
- Add suffix to filename of uploaded images
- Removing Title Tag from Thumbnails
- How to link to the image editor’s Edit Image function?
- Changing playlist shortcode thumbnail sizes?
- insert post & Upload post thumbnail from the front end using ajax
- Regenerate thumbnails after upload
- Front-end Image Upload with Preview – Is this Possible in WP?
- Saving WordPress generated thumbnails in a subdirectory
- Disable wordpress image sizes generation
- Cannot get full thumbnail size using the_post_thumbnail
- Save image as featured image
- WordPress won’t generate image sizes for certain images
- Frontend Post – Allow Only Image File Upload
- Site icons with alpha channel for self-hosted WordPress blog network
- How can I receive the image id using the media box?
- Can I stop wordpress generating media sizes
- Lazyload post thumbnails
- Show featured image next to post-teasers in Genesis Framework?
- WordPress Fancybox Resize Large Image
- How check that there is post thumbnail feature image or not on wordpress?
- If I Regenerate Thumbnails, would WordPress delete the existing Thumbnails and replace them with ‘new copies’?
- Uploading images on front end doesn’t generate thumbnail sizes
- How to get cropped thumbnail source for custom post type
- Can’t seem to attach uploaded image to post and set it as thumbnail
- Media Gallery Upload photo incorrect way like glitch
- Update old Post image metadata to ‘thumbnail’ wp_attached_file
- Attach Thumbnail Generated from Video Upload as Featured Image for the Video
- How to stop wordpress always displaying full-res images
- how to link post-thumbnail to original img in a lightbox?
- Disable image thumbnails for only one upload folder
- Add padding while generating thumbnail
- How to replace images?
- Fixing image names for a site being restored
- Trouble Retrieving Thumbnail URL on Front-Page
- WordPress Image Resize fails without Error Message
- How to force attachment size for every post
- Is it possible to upload multiple images via Media Library tab in Thickbox I’ve created?
- Make thumbnail from uploaded image without plugin
- Set featured link not showing
- Making a thumbnail if there exist a link with. jpg or .png extension in WordPress
- Is there a way to force Featured image to show as attachement?
- WP is showing wrong image dimensions
- Question regarding the file path for media_sideload_image and wp_upload_dir()
- Generate and upload screenshot as featured image for user-submitted post
- How can I add custom sizes for PDF thumbnails generation?
- Trying WP editor to load full size image (not ‘300×300’)
- How to upload multiple images to use as gallery in Kirki Framework
- Restricting Image Upload Sizes using ‘wp_handle_upload_prefilter’ – Stuck media progress bar when Featured Image?
- How do I display a PDF thumbnail as a link to the PDF without uploading the image
- WP “optimizing” PNGs into thumbnails 5X larger than originals – FIX
- Featured Image add tab
- optimized PNGs 5X larger than originals