Forum posting uses a very limited text processor. There is a icon in the editor to input a photo. Using this icon you can change the size of the photo (in pixels) and there is a dropdown to specify location to put the photo (top, middle, bottom etc.). You can also copy and paste your photo. Controlling the photo size is always challenging. If you need to, you can change the size of your photo before copying it by using a third party software like resizeyourimage with this link:

http://resizeyourimage.com/