hello subhajit banerjee - welcome to BrainDen
To make a post go to the appropriate forum, in this case New Optical Illusions and click the "Start New Topic" button. Enter a title then any text you want to include in the large dialogue box. You can attach an image one of two ways. If the image is on the web, you can click the little picture icon above the dialogue box and enter the url. Or you could type the image tags with the url in between like this:
[img]www.my_image.com[/img]
If the image is on your computer, you can click the Browse button below the dialogue box. Once you locate your image, click the "Attach This File" button just below, then the "Add to Post" button below the dialogue box and to the right. That should do it. Please do not hesitate to ask if you have any issues or other questions.