Is it normal that my pictures have their quality suffer alot once they go through ZenPhoto ? I tried adjusting the compression in the settings of ZenPhoto to 100% (best quality). I also uploaded the picture directly through FTP to make sure ZenPhoto would not process it.

I took this screenshot as an exemple of what it's doing.

http://www.xplatinum.org/screenshot.jpg

Can anyone explain to me why I cannot achieve the same quality once on the gallery ?

What colour space is the image in? sRGB, or something else? Try looking at your original picture through your web browser and compare that to how it looks in ZenPhoto.

Just tried what you suggested and the picture is indeed different from my original when opened through the browser locally (both IE and Firefox).

Is there a reason why browsers make my pictures look different than in a image viewer ?

I just found out where the problem lies.

It has to do with ICC profiles. I had it set on ProPhoto RGB. It seems that web browsers do not support them (or at least this one). So it renders the picture without it.

Long version:
http://www.gballard.net/psd/go_live_page_profile/embeddedJPEGprofiles.html

In a nutshell, I'd suggest converting web-destined images to sRGB. I generally do all my editing in ProPhoto RGB these days (and all my old images are in Adobe RGB), but as part of my workflow in saving stuff for the web, I convert to sRGB.

At least here in Europe working with the ECI-profile is recommended for RGB.
A good check how images will look on the web without any profile is saving them with the "Save for web" option in Photoshop.

Just for info: To my knowledge Safari is the only browser that makes use of color profiles.

At least here in Europe working with the ECI-profile is recommended for RGB.
A good check how images will look on the web without any profile is saving them with the "Save for web" option in Photoshop.

I did some searching for this subject here ...
@acrylian: sRGB is the least common denominator and the most compatible colorspace in the workflow from cam via screen to browser or print. Everything else needs much attention and conversion.
The smaller colorspace doesn't matter when it comes to screen display, current monitors can't handle the plus of information anyway. And without an trained eye and direct comparision of properly converted color spaces one wouldn't be able to tell the difference between a print with AdobeRGB and sRGB either, IMHO.
The only thing is, that when uploading originals, the profile for the web-sized images (e.g. 595px) gets stripped off by GD and even browsers with sRGB profile capability won't display them properly.

I do understand the problem but since I am not a photographer nor do I have a digital camera currently I have rarely this problem (I am illustrator and designer actually).

The problem with color is throughout digital media, no matter how the image is created.
If you scanned a sketch and wanted it to display properly on screen you'd run into the same trouble.
(but I'm sure I'm not telling news here).
I'm looking for a fast workflow - so I'm afraid I need to write some PS-Actions to compensate this.
As you stated elsewhere using no profile appears to be worse in the first place but in fact is probably safer (color-wise). I'll test that ...

I now about that of course and the ECI-profile recommendation referred to a recommendation of a color management guide that corrected itself in its newer edition. Well, this color management stuff can be really annoying for sure..