It depends on the clothing item, and how the uv of the mesh is set up. If you add a png with a blank background to the decal slot, you can scale and move it with the tile and offset sliders in the material properties textures tab of the clothing, but you can't rotate it, so if the clothing uv's aren't horizontal/vertical it won't line up. You can also save the uv template with the button at the top right of the clothing materials texture tab to see how it is set up and overlay it in a image editor. It's also hard to do because different morphs will stretch the clothing too.
What clothing Item and image are you trying to do? I can edit a decal for you if you wanna.