I have been looking for this for a long time, so I finally built it. Drag your image onto the web viewer , adjust size and then set the cropped image to a container field. Solution is raw, meaning the files for the web viewer are local with the database but you can host those files on the cloud or into the actual database. Solution was built on Crop it by Scott Cheng. - enjoy!
Download here: http://goo.gl/D6KhGS
- osX 10.11 and FMP v14 tested only.