G Cache Taking Over Updates?

I've done a lot of testing before posting this but I'm quite sure that G cache actually takes over site representation for a given user if they've been there for a certain instance of your test. I manage many sites and was trying to change a graphic image on a test page, of course just replacing it without changing the name and uploading, and after several attempts, even replacing via cpanel file manager, the new image does not show up. I opened up a new site and did a similar test in my developing environment and as usual no problem...I am very curious about this.

Are there limitations to changing files within a page while a test is active???