The way you linked the image is correct, but you need 15 posts to be able to embed images.
The signature needs to be resized before you can use it.
Here are the signature rules for d2jsp:
https://forums.d2jsp.org/info.php?p=9Quote
Signature Rules and Guidelines (violations may result in loss of rights to have a signature):
The entire size of your signature shall not exceed 400 pixels wide by 150 pixels high.
Text sections of your signature shall not exceed 6 lines of text if you include no images, and 3 lines of text if you have an image.
Total size of the images contained in your signature shall not exceed 70 kilobytes.