Set some standards based on the limitations of the images you're working with
As discussed in the comments on Scott's answer, a good option is to choose your smallest image, enlarge it as much as possible without visible quality degradation, and then use the same amount of white-space available in that image as the benchmark for all other images.
Set a standard for both portrait and landscape images
In the examples you've given us, there is one mildly landscape image, and one heavily portrait image. After a little bit of thought, my best advice is to set-up a canvas upon which all images will be adjusted. Make this canvas as wide and tall as an image is allowed to be, without white-space, like so:
Use Save for Web... on each of the images, resulting in:
Then finally, pop each one into a ready made template with guides, and Save for Web... again like so: