I've made all my images 530px longest side, sharpening done myself through the use of GIMP, uploaded them and set the image quality to 100 and no additional sharpening through ZenPhoto. But now I notice an interesting difference when I keep these images next to each other: the one on my computer and the one shown on my website. The image on the website is slightly sharpened!

Any idea why? Does the browser - in my case Firefox - have influence on how the images are shown?

Interessting question. I did not notce something like that and I never heard that browser do sharpening, but there might of course be a difference how certain browser render a page. Like with font display.
What did you view the image with on your computer? Your OS may have a hand in that, too. For example Mac OS X's preview program tends to blurr/antialias everything a little.

You're right... I'm using Linux and checked in GIMP, Eye of Gnome (imageviewer) and Firefox, and it's Eye of Gnome that is softening the images slightly, while the other two show no differences.

Another very unimportant matter solved :=)...

Make sure that in EOG you have zoomed in to 100%

I find that EOG is a little goofy on figuring out how big an image is or isn't, and sometimes zooms its window to the wrong size by a little bit. So before you think that EOG is softening or sharpening the image, make sure you zoom to 100%

later...

Hey jsilve1, this has nothing to do with ZP, but good to know! Never noticed it, but EOG is indeed not at 100% when first opening an image, but a little bit less than 100%.