You can either host them elsewhere on the net and use the insert from link tool on the top bar of the reply box (looks like a little picture) or you can upload them directly to our server using the upload a file button - we prefer the former if possible because our server bill depends on the amount of data stored
You may also have to reduce the file size. At least, I've found photos I take on my iPhone X are too large and have to be reduced prior to sharing.