Just as I thought and sad to say that I don't have any photoshop available in my desktop guessed that I need to find an alternative for that and thank you for that information but one more question and this is just out of curiosity, do you mess around the image size (in pixels/px) just to get the right banner image size? If you do what is the most appropriate banner size for forum? Because I only apply some banners and it adjust to according to the space of the forum.
I didn't mess around with the image size for the images in this thread, simply because when an image exceeds a certain size (I don't know the exact number but I think it's about 1800px width) the forum automatically fits it inside the borders of the thread. My banners were 2016x290px so I didn't need to do anything but if you need to mess with the size of an image you can decide how big you want it to be in the thread.
For example, Normal Banner:
1000px width Banner:
All you have to do is simply add "width=X" (where X is the number of pixels you want it to be wide) inside the image code.
[img]https://i.imgur.com/YOgTZGx.jpg[/img]
[img width=1000]https://i.imgur.com/YOgTZGx.jpg[/img]
I hope I was clear enough!