You would need to reformat them to meet the Character image specifications. Depending on HOW you want to use them, it might be as simple as that, or you might need to move stuff around a bit as well.
Required format can be found in the help file (F1) or by looking at the existing images in the Characters folder. Note - size is not important (but be aware, the sv_actors images ARE larger than the on-map character sprites, so they're going to look weird when used together). What's important is how many columns & rows you have, that each of those is the same size and the sprites aligned correctly, and how you name it.
