Load images only on wi-fi?
There's currently no way to set a system-wide policy for this.  Some apps do support doing things on WiFi only, however.  Here's some info on the apps you mentioned:


You can go into the browser settings and uncheck Load Images.
GMail does not load images unless you view a message and click a button to display them.
I believe the Touiteur Twitter client allows you to disable images.