When you upload an image to a topic it will create a thumbnail for that image and drop it into the topic under your text area. This thumbnail image is set to a specfic size so that it can be viewed online and in mobile devices. Images are presented in rows of three.
To enlarge the image, hover over the image and the hand cursor will appear to let you know it is an active image link. Click on the image and it will open the image to its full size in an image light box.
The image light box size is determined by the original size of the image. If, for example, you uplaod an image 500px by 500px the enlarged image will be this size. If you upload an image 1600px x 800px, the enlarged image light box will be of the same size.
If you wish to reduce or enlarge the size of the light box image, you will need to adjust the image to the desired size before uploading it to the discussion topic.