The way I work round it is to upload anything to Flickr after resizing to a size suitable for posting to the destination, for example this forum has a maximum size and file size limit (I forget exactly what now!) so I usually resize to 900x600 pixels and save at a quality that gives the required file size. No need for Flickr to resize and resharpen so artefacts avoided. Then again, I rarely post shots here anyway since IMO a decent size print is a far better option! I sometimes use the Upload a File button to upload a picture straight from my computer to the forum, again, after resizing and sharpening to suit.