I've never understood why the preview can't be resized either, it's just the preview. Like Riva has done on many of his pics but only one of the pieces has been turned down from that. This place is really werid when it comes to accepting pieces to the gallery, many have been turned down for reasons that apply to other pieces which were accepted. This place is pretty lenient when it comes to rules sometimes, if you make one you should stick to it.

I change preview picture. Dont understand why I cant just resize picture to do preview? I show picture, not preview.

I'm afraid it's not allowed to resize the main picure for the preview. Instead, you should either just use a small 100x100 part of the main piece, or draw something new from scratch. For example, you could just show a part of the sword, or the face, to make people a bit curious. Some people also like to write 'preview' to show viewers that the preview is only part of a bigger picture.

But just resizing is forbidden, I'm afraid.

Yes, I just resize , because picture is too big. 

The main pic has 47 colours, the preview has 408. Did you just resize it with a resize tool?

I was asked to do the same thing. I'm glad I didn't. I couldn't bring myself to do yet another anime character. 

Looks decent, I would just change the blade of the sword, use a reference perhaps.