It is pretty simple, once you figure it out. Next to the "Post Reply" button there is an "Upload File" button. Clicking on this will give you a pop-up window for the file browser, select "Browse" that will give you a standard file browsing window, find the photo you want to post select it and you're done.Ugh apparently I’m not smart enough to post a pic...
There will be an option to show the photo in the post as a thumbnail or a full image. In deference to those who may have limited bandwidth or are using cellphones and paying for data, I usually select thumbnail. This posts a small image that can be clicked on to see it full size. Saves some bandwidth and download time if you don't want to see the photo.
On a Mac, it is not possible, or at least I haven't figured out how, to post directly from the Photos App. When I post, I export the photo first and then upload it. When exporting a photo, there will be options to choose the size and quality of the photo. Again in deference to those with limited bandwidth, I chose large or medium depending on the detail I want to display.