In the Quick Reply / Reply / Compose Message option on a forum; You can see a tool bar just above the "Messeage" window, hoover your mouse over the icons in the tool bar and you'll be able to find a "Host an Image" icon soon enough, like the one in below:


Click the "Host as Image" icon and a Pop up window would open


Click "Choose File" button to browse/choose the pic on your computer. Once done choosing, the name of the file chosen would appear next to the "Choose File" button.

Once done choosing the file, click "Host It" button at the bottom of the pop up.

It will take a minute before the hosting service "servimg.com" uploads the pic you selected. Once image is hosted the pop up would change to the one like this:


Click the suitable address/code option you want to use, from Thumbnail / Image / Image url and copy the code by clicking "Copy" button next to the option you want to use.

Go back to Message Editor/Message Composition Box and paste (Ctrl+v) the code. Below the message Editor, click "Preview" to check if the image you wanted to insert is showing up correctly. If it is, click "Submit" and you are done!