This is basic but... How do I replace an image while keeping the image box dimensions and shape?

If you insert/use a placeholder first, then insert your image into it, I think that will keep the same dimensions if you change out the photo later on. 

Another strategy that I use is that I will manually mark down the dimensions and positions numbers for my images. I try to format my images to have some kind of "standard" size for that project/slide design. For example, my picture format for various icons I'm using at the top of my slide is width = 150, and height = 100. Position is X= 810, Y=10. 

Doing this means that I can quickly change anything new or additional by adjusting the numbers. I also make sure to keep the "lock ratio" option enabled so that it won't stretch or distort my images when adjusting it. 

Oh, and for rounded shapes, make your dimensions of your image exactly the same so it is a square. That makes going to a rounded shape much easier. 100x100 or etc. If it isn't even (100 x 200) it is a rectangle, and you get weird issues when trying to convert to a rounded shape. This strategy should also work for most software, not just articulate. 

Hope that helps a little. Good luck.