Streak doesn't store your images for snippets or mail merges directly, so you will need to make sure your 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
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.
Need a quick way to store your image publicly?
If you don't have your own image host, you can utilize Google Drive for image hosting purposes via Docs.
Open Google Docs and create a new document and give it a name like "Images for Emails"
In the top right corner of the Google Docs interface, click the Share button.
From the sharing window, select Advanced
Under Who has access click Change... from the right column.
Select On - Anyone with the link and confirm the change by clicking Save
Insert the image into the Google Doc
Select the image and copy it from the Google Doc using either Command+C (mac) or Control+C (windows). Then paste it into the Compose window of the Snippet manager using Command+V (mac)/Control+V (windows).
As long as the Google Doc is not deleted from your Google Drive, the image will always be accessible and will display properly when sent.