I've decided to make a small guide to explain how to add images to replies, since I got the feeling some don't know how to do this.
1. Upload the images to a image-hosting-service. We're gonna use https://imgur.com in this example.
1.1. Click the 'new post' button in the top right corner, right beside the imgur logo.
1.2. Now simply drag the image you want to upload next to the nice giraffe or click on browse and choose the image you wanna upload.
1.3. Now that the image is uploaded, you need the image-url, which you can get by rightclicking the image and selecting "copy image url"
2. Add the image to your forum post.
2.1. Click this little icon.
2.2. In the new window, paste the url you copied on imgur into the empty URL field
2.3. You can change the size of the picture by typing in a smaller or higher number for the width and height.
2.4. Click on 'OK'
3. Well done! Now you know how to add images to posts on this forum, now you only need to submit your post.
You can use this to post images in the In-Game-Media Forum for example.
Have fun using the forum and your new knowledge of posting images.