You can change and replace the image of your email template with the following options.
1. Regular template:
Although if your email will contain any image then you have to add those images additionally by uploading with the following option.
Save that image externally in your local system.
And replace this image with your local image as follows.
Click on Browse Servers.
Right-click and upload your image from your local system.
Right-click and select your image which you uploaded.
Click on ok to complete the replacement process.
2. Drag and Drop email template: