Anything in the Characters folder.
By default, Ace expects you to have 8 characters per sheet, and each character has 4 rows and 3 columns worth of sprites. If your spritesheet only has one character (4 rows and 3 columns), you need to put $ at the start of the name. This does NOT include spritesheets that are large enough for 8 characters, but you only have one and the rest of the image is blank - that still has 8 characters as far as Ace is concerned.
The other thing you need to know about is ! at the front of the filename. This is used for objects rather than characters. By default, characters are drawn about 4 or 5 pixels higher than the grid. But it would look weird for some objects to be drawn like that (like doors, for example, where the base of the door should be right on the grid, to match the base of the wall). ! at the start of the filename tells Ace to draw it right on the grid.
So if you need both, you can either put it as $! or !$ - either is acceptable.