Consider Texture Paint mode editing for the 3D Mesh.
In response to your statement/question [apply an image to just specific faces of a UV map?]
Blender texture paint mode can paint directly on the image to be used for texture mapping.
When a source image has transparent areas you can directly paint on target image leaving the previous background.
A sphere was uv unwrapped. Then a material was created with a texture and image source. First a pig image was used as the source in paint brush mapping stencil mode. Next a yellow arrow was a source image in stencil mode.
Save your painted image many times during the process. The image must be written to disk or you will lose your work. Silently.
In this example we painted directly on the 3d Mesh surface, in superior fashion. You may also paint on the image editor surface in inferior fashion.
There is a learning curve for the many features of texture paint mode, and Blender is always full of surprises. I would suggest a video tutorial by searching
[Blender Texture Paint]
mode at some well known video site.
Lastly seams can be added before UV Mapping. UV faces pulled off the image. Texture Image Mapping Extension [Clip] to stop application of texture to a face. This is supplemental information.