In your NiftyImages account select "Create Image Or Timer".
Select "New Personalized Image".
Select or upload your own image. Then Save your options.
Make sure "ElasticMail" is selected as the ESP.
Copy the HTML.
In your Elastic Email template, use a text layout box.
Choose the Raw HTML option, then paste the HTML.
Add some css to auto adjust the image width.
When the template is sent to a campaign, contact's firstnames will be merged into the image.