QR Code Redirect lets you set up your own QR Code redirection site. Generate a QR code for a URL on your site, and redirect that URL anywhere.
Use the following shortcode:
[qr-code id="xx" size="xx"]
where id is the post ID of your QR Redirect, and size is the desired width of the image in pixels. Size is optional, and will default to 250 pixels.
No. The QR Code points to a WordPress permalink on your site. The only time you would ever have to switch out an image is if you change your site's permalink settings, and thus change the permalinks of the QR Redirect posts. Presumably this is something you won't be doing too often, if ever.
QR Codes on their own are static. In order to update them, you have to generate a whole new image (which would suck if you were putting them on flyers or some other printed medium and suddenly needed to change them). This plugin lets you point your QR code's embeded URL to a different web address if you need to. For example, if you are using an off-site service to host a contest, you can point a QR code at that site for the duration of the contest and later change it to point to another page with the contest winners.
Courtesy of Google Chart Tools, specifically their Infographics. http://code.google.com/apis/chart/infographics/docs/overview.html