Gmail does not support the use of HTML in email signatures, so there's no way to have a clickable miiCard card image. However, the following workaround will allow you to include your miiCard in your Gmail signature.
Login to your Gmail account and click on the Settings button to the top right of the page.
From the drop-down list click on Settings. You should now see your Gmail settings.
Within Settings scroll down the page until you see the signature option.
Click on the Add Image button to add your miiCard into your signature.
In the window that pops up, enter the following URL into the Image URL textbox:
Don't forget to replace <USERNAME> with your miiCard username. Now click OK to save and your miiCard will be in your signature.
Your miiCard is now displaying in your signature but it doesn't do anything at the moment. You will need to add a link to your Public Profile page so people can verify your identity. Drag your cursor over the image to highlight it, then click the "Link" button to turn it into a link. In the popup enter the following URL into the Link To text box.
Don't forget to replace <USERNAME> with your miiCard username.
Click OK, then click Save Changes at the bottom of the page to save your signature. This will now appear at the bottom of any email you now send.