New answers tagged image-resize
I don't think there is any all-in-one solution if theme or plugin uses not resized images. They can reach for source images or even access directly without WordPress functions. So, the only way to truly force this kind of themes/plugins is to edit they source files. Which is known as bad practice. But there are some plugins that can reduce this ...
You're right that it just doesn't work like that. If it's OK to think of your question the other way around though, you can get the right outcome in modern browsers using a selection of image sizes and responsive images. If you use code like this: add_image_size( 'custom-size-small', 200, 200, true ); add_image_size( 'custom-size-medium', 300, 300, true );...
If the images are all referred to by URLs embedded in post content, then you could just upload the missing files from an old backup.
If I've understood correctly, you just want older images upgraded to be like newer images? Back up your data first, but if I have understood then just remaking your image sets with a plugin like Regenerate Thumbnails should do the trick. Test this out on a development copy of your site though!
Top 50 recent answers are included