I was checking Imageresizer S3 Reader2 plugin, and I have the following question.
My app is basically a c# REST API that has a functionality of serving photos (resized photos).
Would it be possible to use Imageresizer+Amazon S3 with REST API so I can resize photos in with Imageresizer in c# before serving it and without transferring original photo over network?
You'll have to transfer the original photo from S3 to your server (at least once) in order to resize it. The S3Reader2 plugin does this automatically. If you want to prevent repeat requests, look into SourceDiskCache.
Otherwise, that's exactly how ImageResizer+S3Reader2 functions.