Click on the 'Upload' icon to open the Upload dialog.

Click 'Browse' to locate the image to upload from your local machine.

Check the 'Resize Image' option if you wish to resize the image on uploading. The 'Max Width/Height' value is the size of the biggest side (width or height) you wish to resize the image to.
ie : a source image of dimensions 800px by 529px will be resized to 600px by 396px at a setting of '600'.

Check the 'Create Thumbnail' option if you wish to create a thumbnail of the uploaded image. The dimensions for this thumbnail will those as set in the configuration (default is a max value of 150px.)

Click 'Upload'.