Did you put a "$" at the front of the file names? Since they are singular sprites and not on a sprite sheet the program needs the $filename, rather than filename to properly select a sprite.
$ or ! automatically sizes all character sheets to work.
Not true. Let me remind you that using ! in the filename will only get rid of the 4px difference at the bottom of a sprite. You'd still have to use the full character sheet size of 4x2 characters. It's only when used in conjunction with $ does the sheet get "resized" properly. All character sprites, unless they have a ! in their filename (ex. !Door.png) are all shifted 4px upwards, so when you hit a wall at the bottom of the screen, you can see some space between it and a character. If you put ! in the filename, that is ignored completely and WYSIWYG (what you see is what you get).
The $, however, will assure that your sprite sheet for single characters is read properly and the character are selected properly in the editor.
From the Help File (contains so much valuable info):
Note that RPG Maker VX Ace displays characters offset four pixels from tiles so as to more naturally portray them with buildings.
Adding an exclamation point (!) to the beginning of a file name cancels the application of the four-pixel offset, and also turns off the translucent effect applied by the bush attribute. This is used mainly for object-type characters on maps, such as doors and treasure chests. It can also be used in combination with the dollar sign ($) special character.
Adding a dollar sign ($) to the beginning of a file name allows you to treat one character as one file. In this case, the size of the character will be one-third of the width and one-fourth of the height of the file. It can also be used in combination with the exclamation point (!) special character.