MUCH better to resize it on the workstation... We constantly get folks that take pictures with their 12 megapixel camera and then try to upload them without resizing the image. :: facepalm ::
plupload lets the admin specify either Vertical or Horizontal pixel limitations or filesize limits that any images will be resized to on the user's machine, BEFORE it's uploaded. That's a LOT better having people trying to upload gargantuan images and getting frustrated because it times out, or exceeds upload limits etc. All us web geek types have no problem resizing an image, but you'd be AMAZED how many people haven't the first clue how to do that, or even why it's needed.
The point is, you want to make uploading images as simple and painless as humanly possible. If images go into a centralized, searchable gallery, they can be priceless user generated content. And so many people use their smartphone cameras to take quick snaps of things they want to share, an image is often all they want to post with maybe a few words to go along with it. Forums need to get with the rest of the modern web in how they deal with images.