When you select Flash a properties window opens.
You can use the Select File button to browse for and then Add and Use your file.
You can then change the dimensions by un-ticking the field Use Default Dimensions. You will then be able to amend the height and width. If you tick the Maintain Aspect Ratio you will only need to change the Width or the Height of the image and the original proportions will be maintained.
You can enter a Text Alternative for users who cannot view the image. This Wikipedia article outlines why you might want to do this.
You can then enter the required parameters, entering Name and Value as required.
Click OK when you have finished.