Will need a little more details to help really.
If you mean images inside your posts then you can upload using the Add Media section of the write post panel.
If you mean change the look of your theme like replacing the header logo with your own, then check what the header logo image is called and replace it with your own. It will probably be in the themes images folder somewhere.
Also check out the appearance tab in the WordPress dashboard as many themes have there own custom header section where you can change these things. That is dependant on which theme you are using though.
