Once you upload the photo to a site like imgur or postimages, you should be given a link to your image. Once you have your link, type in the tags. You will paste your link inside the tags.
For example the Kakuro puzzle for this event was uploaded to postimages. The url for the image was
https://i.postimg.cc/mgBQbbrK/picnickakuro.png, to upload it as an image I would type [img]https://i.postimg.cc/mgBQbbrK/picnickakuro.png[/img ](and remove the space from the end [/img ])