To let your clients know they can submit videos, tweets, and photos, simply mention it on your testimonial submission form somewhere. For example, in the field labels in the Fields editor, or in the page or widget content before the
Don't get fancy
You may want to mention that you only need the plain URL, not the fancy "Embed" code those services may provide.
Example plain URL:
Example embed code:
<iframe width="560" height="315" src="https://www.youtube.com/embed/fQDLii4PKJ0" frameborder="0" allowfullscreen></iframe>