Why the header image not consistent

This is the header image showing in IE browser.

This is the header image showing in Firefox browser.

Can anyone tell me why the header image is not consistent at all? In fact, my header image didn't show at first during my trial in Firefox, i didn't change any code but just replace the header image, means the header image still has the same name as the default one. I loaded my page for several times but my new header image still couldn't be shown until i cleared my cache, yeah, my new header image only able to appear after i cleared my cache folder. As for the IE browser, i just don't know why my new header image can't be shown properly. Do you know why? Let me know the solution if you know what the problem is. TQ!