Streak doesn't store your images for snippets or mail merges directly, so you will need to make sure you host it somewhere publicly in order for the image to be rendered correctly in Gmail.
Note: Inserting the image directly into the snippet or mail merge without hosting it publicly will cause the image to break upon sending the email in Gmail.
Adding publicly shared images to your snippet
If your image is already hosted publicly, you can add inline images by using Gmail's Web Address (URL) option in tandem with your snippet.
Open a new Compose window or start a mail merge
Click the Insert Photo button from the toolbar at the bottom of the window
Select the Web Address (URL) tab and paste the URL into the input form
Confirm by clicking Insert
The Loom video below shows an example of how to host an image using Imgur:
Note: When using Imgur as your image hosting platform, please make sure to create an account to ensure the images do not get deleted automatically.