OK, I didn't realize the pop-up images were using the same classes as the profile avatar. I had shrunk the size of the avatar to look better. So for anyone else searching on this, I had to qualify the classes like this:
.cbPosCanvasPhoto .cbImgPict.cbFullPict {
width: 120px;
height: 120px;
}
.galleryImage.text-center .cbImgPict.cbFullPict {
width: 100%;
height: 100%;
}
.gjGroupPhotoImage.text-center .cbImgPict.cbFullPict {
width: 100%;
height: 100%;
}
yes, still learning CSS...