We have created a KB article where a possible approach to achieve image resizing before the upload is suggested. You can find it at https://www.telerik.com/support/kb/aspnet-ajax/upload-(async)/details/resize-images-client-side-with-html5-before-uploading-with-radasyncupload The article describes a straightforward way on how to change the uploaded image dimensions. You can further improve and modify the logic in the _uploadFile function to allow the user to define the new dimensions or use a tool to allow the users to modify the image.