You click on the thingy that says:
> Drag files to upload or click here to select them
It should pop up a window where you can browse the files on your system, select the one you want to upload, then click Open. It should list the file just below the text I quoted.
Then click New Reply and should upload.
There is a filesize limit however, so too large files aren't possible to upload. A size of a photo should upload fine.
It is possible that the Board Owner reduced the allowed file size, then it could fail.
If this process fail: do you get any error message?