Here's a re-cap. Since you already put the photo(s) in your album, the rest is easy.
1) Start your post or reply, just like always.
2) I open my album in another tab to make it smoother, but you don't have to.
3) Find the photo you want to include in your album. But don't just use the thumbnail - instead open it up by clicking on it so it's big.
4) Hover your cursor over the now-big photo and (depending on your operating system) right click or control click or etc. and then (on a Mac) choose "copy image location." On a PC I can't quite remember but I think perhaps choose "Properties" and then inside that you will find the image location data (which is what you want). Copy that.
5) Come back to your post and paste the info from step 4 into your post where you want it to appear.
6) Highlight that string of characters.
7) Go up to the area above the white writing box and you'll see a row with B, I, U, etc. Go across until you see "Img" and click on that. That will put the appropriate tags on each end of the string of characters from step 6.
8 ) Optionally, if you see the word "sized" right before the .jpg in the characters in step 6 and you want the image to be even larger, you can eliminate the "sized" part from the character string ("sized" makes the photo the smaller version of the two larger versions).
9) Finish your post.
10) Instead of "Submit" hit "Preview" to check things out. If the photo doesn't show up properly in Preview, it won't do so in the real post either. When there is a problem, I usually find that it has separated the ending image tag from the line of characters with a line break or etc. I backspace to get rid of that and Preview again, and 99% of the time that fixes it.