I have a lot of experience with this particular tool-set, as I tended to include videos and images frequently as part of in-class lessons or online tutorials with my students. My favorite sites for gathering videos to use in the high school classroom were PBS, NPR, and YouTube/TeacherTube/SchoolTube.
One thing I've learned over the years, when it comes to using videos in any sort of live presentation it is best to download the file to the computer. I learned this during the year when my students created their own lessons to teach over various topics, and many of them included video clips. The problem was, clips that open at home don't always open at school because of firewall issues. I found it easier to have students email me links to their videos and I would download them to my computer in the evenings. Presentations all ran from my computer, so the files were ready on the desktop.
For Mac users, consider purchasing the app Wondershare-AllMyTube. It is about $30, but it makes downloading and converting videos from the web SO easy, especially if you do it often. I used this program over the summer to allow me to imbed videos into an Adobe Connect online discussion room.
As far as images are concerned, I prefer to include my own photographs whenever possible. I found Creative Commons to be very useful when I was selecting generic images to use in some of my teacher presentations. I'm probably guilty of some copyright infringement in my classroom lessons, but I am always careful not to publish anything on the web that is not 100% mine.
I have used Dropbox, but it is not my favorite. I prefer to upload files and documents to a classroom wiki, Edmodo, or Google docs for sharing purposes. I find that if I use too many filing systems, my files tend to get lost in space. :)