If you don't have PhotoShop to resize, you can download Gimp which is open source software similar to PhotoShop. Here's the Gimp link http://www.gimp.org/
and here's a blog post how to resize an image using Gimp (I am sure you can find a tutorial too if you Google). http://www.novell.com/coolsolutions/feature/11503.html
You can also resize in FotoFlexer http://fotoflexer.com
without downloading any software.
You will need to know the best size to resize the photo from the template you are using. If you don't know, you may need to experiment with the sizing. Most template sites have a community forum for support. If you check the forum, you may find the answer you are looking for. If not, you can post a question there or in most cases you can contact the template designer and ask for the size that you need. Also check the user documents that typically accompany a template purchase for the correct size for the header.
I've used Logonerds for a few projects with much success. Let Logonerds know the size you need and they can resize the final copy for you.