It's harder to explain than it is to do but I'll try.
In your image gallery post right click on the pic you want use , then click on "copy image location". Now in the thread you want to post the pic, click reply then click on "use full editor". Just to the right of the smiley face is the box for insert image, click on that and paste your image location in the box . After that click OK. When you post that reply the pic should be there, like this.
If you have multiple pics you want to post, I just open 1 tab in image gallery and 1 in the post I want to paste them in and just move between the 2 tabs to move the pics.
Lee