Adding an image, or a link to an entire album, from the gallery into a post or story is easy.
If you want to do more than one image, it is usually best to have multiple browser windows, or tabs open, so you can have your post going at the same time as the gallery.
First, navigate to your desired Gallery. You can embed a link to the whole gallery, or to a specific image.
From the gallery view, when you want to link to the entire gallery.
Note the "link to the page of the item" block?
Highlight, right click, copy. (or click, then control-C)
Now, back at your forum post / story / whatever.
Right click, and paste.
Since this reads "click to view image" you would do well to change that text to something more meaningful, like "Photos from our run"
Now, want to include a photo?
Find your photo in Gallery.
While looking at the photo, examine the URL block on the right, and make your choice.
and paste again
The actual syntax various slightly depending on which option you picked - the thumbnail linking to the original, the resized image, or others.
Avoid linking directly to BIG pictures - it won't look pretty. You should instead use the "clickable resized image linking to the original" or clickable thumbnail.