You can do one of two things.
When you have uploaded your photo and you see the copy link box, click on the drop down arrow, then click Get Shared Links, I find the Markdown (Reddit) link works best, copy that and enter into your post.
OR
Once photo is uploaded go to your username in the top right corner, click images, then click on the image you want to use, another box opens then I select Direct Link, copy and paste into your post.
For some reason the direct link doesn't work from the first option, not sure why but both of these ways work for me.