BuddyPress.org

This is just off the top of my head. No guarantees. Try something like this:

<input type="image" name="search-submit" id="search-submit"

src="/path/to/your/submit.gif" alt="submit" />

Please realize that since you are hacking the core (making changes to a core BuddyPress file), the next time you update BP, your changes will be lost. Instead, you should create an alternate function for bp_search_form() and place that in a bp-custom.php file.