You must have an isolated image layer in your file to add this as a variable. After you log in, click 'Add new variable'. Then click 'Select the correspondent layer' in the drop-down menu to choose the image layer. Fill in the requirements concerning this variable (variable name and optional/required variable)) and click Save.
To add an image variable, you must include a file within your project.
Go to the Variables tab, then select the image layer from the drop-down menu. Set a name, check whether this variable will be mandatory or not and whether the Image Fit feature will be used.
Then just click on 'Add Variable' and it will appear in your list of registered variables.
Please note to remember to collect the files before loading the template to ensure the file will find all external files.