There is a nice undocumented function Preload on the WebPicture class in Real Studio 2011r1. You can use it to load a picture to the browser without displaying it. This is very useful for a slide show where you want to have all picture local. So your code has this steps:
Create picture somewhere
Call Preload to load it. (e.g. in webpage.shown event)
Use it later with some other event like a button action.
As the picture is preloaded, it will show instantaneously.
Check in Web Inspector (Safari) to see when picture is loaded to the website. There is a similar extension available for Firefox, I think.