Sure! The header image in the Bouquet theme is setup using the CSS "background" property, and so it is something you can change.
The only caveat in this case is that the "background-size" property is CSS3 and so it's not supported in all older browsers, like IE 8. You can see this browser background compatibility chart:
http://www.quirksmode.org/css/background.html
In Appearance -> Custom Design -> CSS, you can add the following to your CSS Editor, then click Save Stylesheet, and it'll work in all up-to-date browsers (I've added comments for reference):
/* for the responsive header image */
#branding {
background-size: contain;
background-position: center;
background-repeat: no-repeat;
}
@media (max-width: 1140px) {
#branding, .header-link {
height: 120px;
}
}
@media (max-width: 860px) {
#branding, .header-link {
height: 100px;
}
}
@media (max-width: 650px) {
#branding, .header-link {
height: 80px;
}
}
@media (max-width: 450px) {
#branding, .header-link {
height: 60px;
}
}
/* removes border for the responsive header image */
#branding {
border:0;
box-shadow: none;
}