It is always the best practice to start with an image larger than your final size.

Here are blown up sections of 2 images.  One started out as a high resolution image that was scalled down to final size of 18″ x 12″.  The second started out as a low resolution image that was scalled up to a final size of 18″ x 12″.

To achieve the proper resolution for images that will be part of your poster, look at the following guidelines.

Open your image in a photo editing software package.  For this example I am using Photoshop.
– Choose “Image”>”Image Size”
– Highlight the Width dimension and choose “Edit”>”Copy”
– Highlight the the “72” in the Resolution field and type “300”
– Highlight the “Width” dimension and paste in the original number

– Click on the down arrow in the “Pixels” pulldown menu and change it to inches

– Lastly, type in your final image layout size.  Remember, it should be smaller than the exiting number.