Hi there!
WP Offload Media Support Team here, Thanks for reaching out with your query we would be happy to assist
1# if I purchased the offloaded media tool plan, all my existing media can be offload to my amazon s3 bucket. Will old image url be automatically replaced with new amazon s3 bucket url? Or I still need to mannually update image url in my site element?
Yes, as long as those existing media files are also uploaded on your Media Library then the pro plugin should be able to offload all those files to your bucket. Once offloaded, it will automatically rewrite the local URL to use the offloaded URL.
https://deliciousbrains.com/wp-offload-media/doc/developer-guide/#automatic-offload
WP Offload Media tries very hard to be a “good citizen” and maximize compatibility with WordPress and third party themes and plugins.
To accomplish this, the plugin does not update any content stored in the database to use cloud storage URLs. Instead, it dynamically rewrites native local media URLs to cloud storage URLs as they are retrieved from the database.
https://deliciousbrains.com/wp-offload-media/doc/developer-guide/#rewrite-urls
2# another question is, once I upload one single image to s3 bucket, it actually generate same image but multiple size in s3 bucket. For example: if I upload a image called test.jpg via wordpress media, there are list of images called test-1024×576.jpg, test-1067×600.jpg, test-1170x785x.jpg…. in the end, if I want to upload one image there will be 15 new images with different size saved in s3 bucket. how can I avoid that.
Generating multiple image sizes when you upload an image is an expected behavior of WordPress and it is done automatically. Your theme may also create copies of additional image sizes.
When it comes to WP Offload Media’s offloaded media limit rest-assured, an image uploaded to / offloaded from your Media Library counts as 1 item toward your limit regardless of how many resized images are generated.
However, if you intend to prevent WordPress and your theme from generating image sizes, you can refer to this doc –
https://wpengine.com/resources/prevent-wordpress-image-size-generation/
Hope this helps!