I go to the photo in my album, click on it to open it (so you don't end up linking to the thumbnail), and then control-click on it (which I think is right-click if you are on a Windows machine). That last maneuver brings up a sub-menu onscreen which has an option of "copy image address" (maybe "properties" on Windows, but basically the same thing). I do that (copy address) and then paste that address into the typing-a-post box, then go up above to the "Img" tag box (just above the typing box) and click that to put image tags around the address.
Then I Preview the post to see if all is okay - sometimes I find that some little issue causes the image to not be correct - for example, the final image tag has been put on the next line. I then fix it and Preview again, then hit Submit when all is good.
So, in summary:
Go to photo in your album
Open it
Right click and get address, copy that
Paste it into your post
Add image tags (manually or by clicking "Img" above)
Preview, then fix any problems
Submit
Maybe someone else has an easier way, but that's how I've been doing it.
Sunbeam