I've been meaning to post this for a few days, and J&K's e-pics post reminded me to actually do it.
1. Right click on the image and select "Properties."
2. Copy the image's URL.
3. Paste it into another browser (window or tab) in the address field.
4. Change "Medium" to "Large" in the pasted URL.
5. Win!

So the URL for this guy ^, is:
http://sitelife.theknot.com/ver1.0/Content/images/store/
8/3/e8d125ec-7bf0-47d5-9303-624369f17011.Medium.jpgWhen you replace
Medium with
Large in the URL you get this:
http://sitelife.theknot.com/ver1.0/Content/images/store/
8/3/e8d125ec-7bf0-47d5-9303-624369f17011.Large.jpg