Part of the web page being MODIFIED !

[Frank Cheong <chocobofrank () hotmail com>]

I got one serious question that is I received complains regarding one of 
the image on my web site has been modified by a PORN picture ! While the 
image have resumed normal during the second visit.

After receiving the complains, I have of course double checked the GIF 
image's filesize and date timestamp found that it is completely normal. 
Therefore, the image haven't been modified. So I do want to know what is 
the possibilities in doing this ?
(Like HTTP session hijack, proxy poisoning, someone doing man in the 
middle etc) any other ways to do that ?

As these activities mostly happens outside my server boundry, I assume I 
can't do anything with it, how about any outside parties ?

As I know going for SSL maybe one of the alternative to stop this but this 
will add on extra processing on my website and it will make it slow. So I 
don't want to go for it, any other way to secure against this ?