Oh in fact I didn't ask the good question (sorry for my poor english).
The problem isn't the size but the weight : my creative file for the header is around 100Ko and Ning reduces it automatically to 45Ko, so the picture is quite ugly in the end.
So it's the same question but about the weight. Sorry for the mistake and thanks for the answer :)
Thank you Susan. The file is a jpeg and I use photoshop so I can adjust the compression level. My question is really about the maximal weight for the header image so that Ning doesn't reduce its weight with its script.
With an image of 100Ko, Ning makes it a 45Ko image and it looks awful...
Well I can use photoshop to make the image lighter (like jpeg quality=20), but it will not look as good as the original file (jpeg quality=100).
So the question remains the same: is there a way to avoid the auto resample script from Ning with a light image, and what weight should it be? I'll perform some tests and let you know...
The off site image option would be something great. You can already do this with the text box and it would be great to have this for the header one day :)