I had the same problem mthmob described (and it was not caused by a contact synchronizing problem).

So I tried to define a seperate Skype contact picture into the N900 as well - this can be done when clicking on the picture area in the mask where you can activate the skype profile. I used the same high quality picture I used in my Skype profile via PC as well.

But the result was worst:
The N900 is downsampling this image to a very very low thumbnail quailty and now uploading this poor image each time to my Skype profile when I'm activating Skype via the N900. And the bad news is that this cannot be removed from the N900 via GUI.

So please can anybody give me a hint how to solve this problem?
For me it seems like a bug since the N900 should either take the contact picture of the phonebook contact with the same name or handle an other given picture properly without changing the size/quality.