Sometimes in online galleries as e.g. Flickr, there are thumbnail previews in the page you are grabbing your image from.

When you select one of those by mistake (after all they all seem the same when trying to grab one of them for your link) it will be rescaled upwards resulting in loss of clarity (blurring).

In order to select the right thumbnail, you should take into account the small number underneath each of the thumbnails.

This is their pixel resolution and the bigger the better (see attached image).