Why sometimes, some images on my website aren't being loaded by a browser

I'm developing a web app and during the tests I noticed that sometimes browsers don't load all images (like background or some decorative images) on the website. I need to hit the refresh button to to have it fully loaded. This usually happens when the website is visited for the first time.

I suspect that maybe the development server is not fast enough and browsers "assume" that the links to images are broken because there is no fast response from the server. But I'm not sure. At the moment my development server is on a remote host inside VirtualBox with Ubuntu-server 8.04. I know that it is not the开发者_运维百科 best configuration, but I'm not sure whether this is indeed the case. Or maybe it is due to some apache/php configuration parameters?

I would appreciate any suggestions or clues. Many thanks