"Choose File" if you are using a Google Chrome browser. It will usually follow the words "Upload embedded file" on Chrome. Generally the wording in the box depends on what browser you are using. If you're using Microsoft Internet Explorer, it will say "Browse". If you're on an Apple product, I can't tell you exactly what it will say, but there should still be an option with an Apple browser.

In each case though, you get taken to a place where you have stored images that might be used on the board and you can select which you want. You can also select a different folder on your PC if that's where the file you want to use is located.

If you want to verify that you got the file you expected, I suggest you "Preview" before you "Post". You can always change it if you find an error before you Post.