Yes you can; you are the copyright holder so you can apply whatever license you want.
However, I recommend that you avoid the licenses designed for source code (such as GPL, BSD and so on) and choose a more general-purpose license. The Creative Commons licenses are an excellent choice; they have been used for books, photographs, music, movies and many more things. Flash videos are surely no problem.
The problem with source code licenses is that they can cause confusion, and worse, some like the GPL may not even work since they require access to the source code for binary distributions.
Since you want to integrate these files with an existing open source project, it's important to make sure that the licenses will be compatible. Fortunately most licenses do not consider binary files (like videos, images) as critical parts of the software, so most licensing choices should be safe. For example, people have licensed projects composed of GPL source code + proprietary assets.