I have gotten used to saving two copies of a photo when I am finished editing - one I would use for personal printing and one sized for posting here ( 1200 px on the longest side as @Fujidave said above ). I have a flickr account but have never used it to post. Uploading from your computer is simple. I suppose if you are interested in promoting yourself for business reasons, posting through a site like Flickr would have it's merits since you would be putting your body of work out there for a potential customer to browse.