Yes you can improve the image quality. The default DPI for ODS PDF output is 150 DPI. Each image you provide also has its own DPI. Depending on what version of SAS you are running ODS PDF will attempt to resize your image so the output DPI and the image DPI match.
To find your image DPI look at your properties. Find out what the horizontal and vertical DPI settings are. Remember the higher DPI the more clear and detailed you output and image will look.
I'd love to help you more but I need a little more information. Technical Support can help or you can email ods@sas.com and I'll look at your image and we can find a perfect DPI for your needs