According to Kawashima (writer of the blog), some graphic formats will be changed in RPG Maker VX (oh no!). Appearantly some will have different format to RPG Maker XP.
They retain the same size and format but minor adjustments will be required because of the new autotiling feature.
RPG Maker VX's character sets will resemble that of RPG Maker 2000/2003; each file can have up to 8 characters in one set. Looks like they are bringing back old features... Yet again.
However, do not worry as you can still use single characters.
In addition, a new screenshot has been posted that emphasizes the expression bubbles (does it take the form an animation, or a character though?).
Facial expressions brings your characters alive - now that is done easily with RPG Maker VX. The screenshot below shows us the corresponding face graphic in the dialogue and the character's expressions shown in the expression bubble. This will reduce much workload for developers.