It's not that I can't resize them en-mass, it's just that I often crop some of my firearm images, like my little friend above, to keep filesizes down, extraneous content out, and still maintain some semblance of resolution. So the SAR-1 image above was cropped quite a bit from the original Canon PowerShot picture, which is why you don't see my document shredder on the end of that power cord on the right, nor is the top of the secretary's desk visible, etc.
So were I to resize such images to a fixed standard, I fear I would introduce distortion after I already made them a custom dimension via my amateur cropping efforts. For those oddball sizes, I'd just as soon keep them in those dimensions, and just add the watermark manually.