The image of your band or your own image is the way you look, the way you sound and the way you are in the public. If you have an appealing image, people will probably think you're interesting and maybe go check out your music. If you don't have an interesting image, you will have a harder time letting people know that you create awesome stuff.
As Chris Robley says in an article on diymusician.cdbaby.com1:
"Well, your brand is about lots of things: your look, your sound, your vibe, etc. It’s not something you can just present to the world overnight. You have to build it, both for yourself, and for the world. But I think if you start with a clear vision for what your music should sound like, what your band should look like, and how that works with your logo, album art, website design, etc, — then you’re off to a good start."