Somewhere in your PrestoPhoto gallery profile there is an option to allow comments. BUT, I must be more exhausted than I thought because I can't find it. Hopefully someone will come along soon that knows and can tell you. In the meantime, I will keep looking.
OK, found it. Go to your gallery and across the top of the gallery where you pages show there 9 little icons on the top right side. Go to the one that says "Edit" (rectangular shape with a wrench on it between the filing cabinet and circle with 1:2 on it). Go down to the option that says "Image Comments" and check the box. That will allow people to leave comments on your photos and pages.
OK, found it. Go to your gallery and across the top of the gallery where you pages show there 9 little icons on the top right side. Go to the one that says "Edit" (rectangular shape with a wrench on it between the filing cabinet and circle with 1:2 on it). Go down to the option that says "Image Comments" and check the box. That will allow people to leave comments on your photos and pages.