The site will automagically (technical term) resize to 900x as an attachment.
It will also resize as you upload to your gallery.
If you point off site you can only have max width or height of 1024. I think it will resize for you as well but allow a click through, but it can't handle photos bigger than that (I guess, I'm just poking around trying to link to: http://www.thesamba.com/vw/archives/lit ... bile/1.jpg
So the gist is you can point the uploader at photos larger than 900x900 but it will resize them for you. Or you can link but only up to 1024x1024.
It saves bandwidth and also keeps the forum threads from getting destroyed by super-huge images.
I just finished some modifications to Gallery so you can link easier, need to write it up, but you can experiment with the thumb and photo tags and find the links in your gallery, along with the fixed IMG tag.