Start new thread or click "Go Advanced" to reply
Write your post then pass the submit reply button and find+click manage attachments
In the upper right corner click "add files"
click browse and find your image, click image and click open
Depending on the size of the file, you may be able to add more by clicking the + button and repeat process. If they're bigger do one then come back to this click upload
You will see each image uploaded show up in the bottom box. Once you have all the images you want uploaded there click "insert inline" at the bottom right, then click done
You will be taken back to your post where you'll see attach codes. You can click "preview post" to see how they'll look before submitting the reply/post.