Replacing Images
It is not always possible, or easy, to change the images used in a website template. Generally you should check with the designer which images, if any, can be changed by you, and which would need to be changed by him/her (resulting in a customization fee).
If a website template contains a main image that is easy to change, or the designer has built in specifically the ability to change the images used in a flash animation within the template, the template should clearly state so.
Some images will simply be "placeholders", such sample product images in a catalogue layout, to show you where your own product images should go. Obviously these can be changed.
Good website templates come with the Photoshop or Paintshop PSD file used to create their design. This is a graphics file that you will need either Adobe Photoshop or Jasc Paintshop Pro to edit. You can then customize the template at will - changing colours and images within the PSD, and then re-exporting them. You will, however, need some experience with editing a PSD file, and will need to be familiar with the concept of cutting a layout up into slices for rendering as HTML.
Keep in mind copyright restrictions when using photographs or other graphics - if the images are not your own, ensure that they are royalty-free and that you may use them on your website. You should also resize and optimize images, as appropriate, to make them as small as possible (in terms of file size). This is to limit how long a visitor has to wait for your website to download.