Hi. sorry for the delay - I did get your e-mail but I haven't had a chance to answer it yet - so i'll do it here
All R4's scenes and announcements are just text files - stored in R4/data/predefine. To make a new scene you just create a new file (probably by copying an existing scene and changing the name and other text in it).
I don't think you can currently use videos in announcements (but I can't remember!) so you have to make a new announcement for it. For Announce video you'd be best off copying the 'announce image' file. I haven't personally tried this, but if you change the text 'TEXTURE' right up the top of the file to read VIDPLAY it may actually just work right away. You then just have to type in the video filename you want to play in the announce screen for it in the web interface.
As for vidcap scenes, you just find a scene that has been posted on this forum and copy it to a text file (with the .r4 extension) that you put in r4/data/predefine. There aren't many extra vidcap scenes around though. I know people have done quite a lot but I think they've been done by pro users who want to keep them to themselves
announce images is easy if you can use the web interface. Just put your image files in the r4/data/tex directory and the type in the image filename in the 'text' box you get for announce image,
hope that all helps,