Yes you could be right - the images aren't all that similar... but then again I have photos of myself that look completely different...
The problem in trying to identify the photos is that I suspect that at least one of them may have been submitted online by someone who has themselves passed on..
It seems at least that the dating of the photo is something we can work with - circa 1860 ... so it is definitely not any of Agnes or William's children as older adults... and there are no siblings in Australia of either of them... and no parents...
The only other possibility would be if someone has mixed these photos up with another family entirely. There are many descendants of Agnes who have these photos up on ancestry - again... one person puts it up and every man and his dog copies... so tracing where they came from originally is difficult - and even then, the person who came across the photos initially may not be sure where they came from...
So we can at least say these photos are "believed to be" of Agnes and Agnes/William.
The baby could also be in fact their eldest grandchild who was born the same year as their youngest child.
Thanks again for all the comments, all interesting and helpful