Just above the text box where you are writing your Post, press the following button:
You can choose from recently/previously uploaded images, but you can also upload a whole new image to your albums.
To do that, press "Upload Image" in the window that opened for you.
You can ignore the step "1", leaving "Post Images" as the default album.
Select the image from your device's gallery/files by pressing the button "Choose File" or "Browse" in step "2".
Press "Upload Image", waiting a few seconds until the image is uploaded to our servers, and then the code should appear in your new Post text.
When you submit your new Post, the video should appear for everyone.