There are a couple of ways to resize the photos. I've taken all my photos on my iPhone. From the phone, I send them to myself on email, which includes an option to pick the resolution (size) of the photo. I use the "small" setting. I download the photos from the email into my computer and from there can pull them onto the website. This is kind of clunky path but this way I end up with the photos on my computer instead of overfilling my phone's memory. This is an OK route for handling a couple of pics at a time but probably not so good if you're looking to post a large batch.
Another route is to use a server-based service like photobucket, then all you have to do is post a link to the photo.