I use a website called PicResize, which I won't link here because it'll be classed as advertising, but you can search for it with no trouble.
Once you select your image either via image upload or URL, you can crop your picture as well as select a custom size. Once you've done that, download the file and then upload it via a conventional image uploading website.
If this is considered advertising, I apologise. I'm referring to it as it is more tailored to a user's needs as opposed to a simple resize that may or may not fit Haiku's specifications.