How to add image？

Are you referring to our Images system? IF so, there's a default limit of 2GB per limit. If the files on your Linode's disks are taking up more than 2GB of space then you may not be able to create an Image. That being said, if you open up a ticket and let us know why you'd like to have a larger image we'll see what we can do to help.

If you're referring to something else, let me know what problem you're running into and I'll do my best to point you in the right direction.