Dots in image

When I make Post Thumbnail image out of a picture from my blog text it gets dotted and the first part of the text from my blog is added; what do I do wrong? What I did was this: Added the Post Thumbnail editor as a plug-in Open the post Choose 'Set featured image' Point at an image and click 'Set featured image' Click 'update' to the post And then I have a nice broad picture on the front page full of dots and with the first part of the post's text Heeelp .... somebody? Thanks, Marianne