In case you want to implement such a compression there is an easy extension point where you can plug in custom logic in order to achieve the desired result.
You have to inherit Telerik.Sitefinity.Modules.Libraries.Data.OpenAccessLibrariesProvider and override the method UploadImage. There you have the new Image object, the stream and the mime type of the content. You can apply any compression you want on the stream and then call the base UploadImage method. To get your code working, you have to register your provider. Go to the backend -> Administration -> Settings -> Advanced -> Libraries -> Providers -> OpenAccessDataProvider. In the field ProviderType, enter the namespace + class name of you type and Click Save changes.
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