As everyone else here is saying, you can not upload a photo to the forum. You could use something like google drive or dropbox if you set the permissions to allow everyone access. You could (as has already been suggested) create a space page, if you have a facebook or similar account you could use that but ideally (in my view) you would upload the photo to a wikitree profile (such as your own) even if the connection was highly tenuous. Presumably if you are posting in the G2G the issue is genealogy related. If the post (and photo) need to be temporary you can close the post and delete the photo but if there is an outside chance it might come in useful or be of interest then as far as I can see it would be doing no harm and might as well stay there.