This doesn't really make a lot of sense to me, as you're dealing with 2d graphics which aren't affected by quality of the video cards (unless we're talking about some seriously old-school 256/16k color cards). Do you have a link to an example? It wouldn't be DNN 'rendering' the image either, a JPG / GIF / BMP has all the math applied to it during creation (compression, encoding, etc.), DNN does nothing else to the image except display it like any other IMG tag, unless of course you are using some type of 3rd party module that 'processes' images (say, a photo album that resizes on upload or an image rotation module (flash, silverlight, jquery, ajax, etc). The only other thing that comes to mind (and it's admittedly pretty late and the grey matter is a bit pudding-esque) is the issue of I.E. rendering PNG files differently. This has been corrected in IE8 but I still wind up having to strip out extra Gamma Level info from the flies meta data or else I get a subtle color variation between IE and FF (most notable when the color matches a nearby JPG or GIF).

Good luck!

Edit - one thing just occured to me... if you are taking a large image that is stored on the server as 2000x1000 (example) and then changing the size in the Image Properties dialogue, either larger or smaller or even worse - irregularly, this could result in some inconsistency being displayed across different browsers. Ideally, you want to make sure your images are sized correctly upon upload and avoid setting the size in-line.  This is a very common problem I have to re-teach to many site editors as many will upload a digital picture straight from their camera at 3000px+ wide and then 'setting' it to 300 in the properties dialogue. This results in the page loading obnoxiously slow (~1meg+ per picture) and the server browser just creaks to a halt when you start getting this in your directories.