The trick to posting images to many of these online hosts, such as Flickr and even FB is to make sure that the image you post will not then be resized by the host. For example I took to posting to FB at 700px wide, as this was small enough that it would not then be resized. I post all my Flickr images at 1024 px wide, and normally view them at that size. Also I can attach them to posts and maintain the full quality at least within the limits of the sRGB colour space, and 8 bit compressed JPEG. Personally from LR I save at quality setting 80, which gives a good level of file compression (40-60% smaller than Q100), but when compared by eye is indistinguishable from the maximum quality version.
If you are posting a slideshow online that usually needs you to be running it on your own server, so the image quality will solely depend on the quality of the files that you save/post. Posting full size images is also pointless as the image would otherwise need to be resized for display, so use sizes that can be displayed full size on the average monitor/display.