Image link in functions

[AngieATC]

I have a really dumb question and I am a bit frustrated with working on this theme.

If I am wanting to use an image in my theme but I want to use it through the functions file do I have to use the complete link for the image or can I just use /images/filename?

I have tried using the /images/filename.png but I cannot get the image to load.. I am trying to make a premade template so I would really like to be able to load the images in the images folder and then have them load without having to use the full link code because it will change when someone uses the theme on their own site.

I really hope this makes sense.