If the image will be used larger in the poster then the original, it should be enlarged when scanning to the size it will be used in the poster. Scan the original at a higher magnification or resolution to make up for the size difference. Do NOT use WordArt. If you insist, do so at your own risk. Do NOT use semitransparent fills.
Results can be unpredictable.
You'll want to maintain good contrast between the background color and the text color. Consider using a light colored background and dark text. For good design, layout the poster into three or four columns of text depending on the poster size. Insert graphics and photos within those columns to add visual interest. There are some situations where you it will be appropriate to move outside the column format.