I don't know if it's possible to move an iPhoto lib (does it properly updates all pathes?)

You can ensure your library is still consistent regarding pathes:
in AlbumData.xml, at the very beginning, you have the 'Archive Path' followed by the value, which looks like '/Users/isaac/..../iPhoto Library'.
Then, further in the file, for each picture, you have the thumb, original (sometimes) and image file absolute pathes: they are supposed to all have the same root path: '/Users/isaac/..../iPhoto Library/.../image.jpg'.

Now, here's how I build my path to access a picture:
When you define a library in wipha, you give an absolute path (which in a standard case, is the same than the AlbumData.xml 'Archive Path'.

I remove this root path from every image path (to reduce memory usage). Then, to access the photo, I add again the root path (the one you set in the admin page), and the remaining path of the file.

So, if you set the lib in wipha to be '/Users/isaac/Pictures/iPhoto Library' but your library contains pathes such as '/Users/isaac/Sites/myPhoto/iPhoto Library', it won't work. In that case, just give the real path in wipha admin.
(but, strange than only some pictures don't work if the pathes are all consistent)

And again, I don't know if you can easily go back to a standard situation by moving your lib. Maybe you can try to burn the entire lib on a DVD, create a new lib at the right place, and import the DVD in it ?