Well, what do you do on your website with it? Per se there is no such thing as "transparent website", but of course they will correctly use the transparency to e.g. stack page elements in separate, overlapping DIVs. Sounds more like you need to work on the website's code to achieve the effect you want...
Thank you for your reply and help.
I forgot to precise that actually when uploading pngs with transparent background made by my friends, they turn out ok on the website, with no white instead of transparent. So I guess the problem comes rather from me.
I tried also to upload my image on another website - a blog running on wordpress, and it's the same problem. Arg!
So I guess it's not because of the website's code ?
Thank you for further help!
Your PNG is definitely transparent, I did a test here against a pink background and it worked perfectly.
What browser are you viewing the page/PNG in? If you are viewing it in Internet Explorer 6, transparency in PNGs is not supported (though there is a fix available).
Otherwise, if you can post a link to your web page, I can take a look and might be able to help.