Not quite sure I understand your question. You took a picture (with new cameras, I'll venture a guess that it's over 500kb). You want to get it into Aperture as small as possible?
Why don't you insert the original into Aperture, and you can make a B&W there, crop it, make it as small as you want, and then export it?
EDIT: Oh, you mean to post it here.... Gotcha. If you have it up on flickr, just get the image address (right click on an image, then select "copy address". When you post it here, just select that yellow box with the mountain, and past the address there. It doesn't have to be that small.