There's actually a process you need to follow to make it work.
Firstly, you can't just plop it in the folder. You need to go through the Resource Manager window and Import it into the respective section. You also need to make sure that the image fits the requirements for the engine; I trust you didn't alter the dimensions of the image, but if you did, you're gonna need to change them back. You can check the Resource Specifications section of the Help glossary for information on proper file dimensions. Then, it needs to be either a .bmp or an 8-bit .png. Because .bmps suck, have huge filesizes, and screw up the colors of the image a bit, I definitely recommend 8-bit .pngs for imported image resources.
Paint.net is a great image editor that allows you to convert files to 8-bit .pngs without losing any colors, provided you're not trying to import a really, really detailed image that uses lots of shades. Once you've got the image set up, go into the Resource Manager window, Import it under the BattleCharSet section, and make sure to highlight the background color as the transparent shade (click on it so that it starts flashing before finalizing the importation).
Secondly, you'll need to set up an entry for it in the "Animations 2" tab in the Database. You can overwrite an RTP animation set or put it in a new slot; it doesn't really matter, but don't overwrite anything you plan on using for something else, obviously. If you're having trouble getting the animation set up properly, referencing how the RTP animations are set up should help.
Once you've got all that done, then you can assign your custom battle animations to a character or file via the normal method.
