First thing's first: you'll want the e-mails your Magento store sends to customers to use your store's logo, so you will need to configure this in Magento's control panel.
At the moment, the order confirmation e-mail will look similar to the following template, using Magento's own logo and a placeholder store name if you haven't configured your Magento store fully yet:
Once you are logged into your Magento administration panel, navigate to System | Configuration. From here, select the Design tab in the left-hand column, as shown in the following screenshot:
Expand the Transactional Emails panel, and you will see that you are provided with two options:
alt
text used for the logo.Select the logo image you wish to use, and populate the Logo Image Alt field with a suitable value, as shown in the following screenshot:
Once you have done this, click on the Save Config button. If you now cause Magento to send an order confirmation e-mail again, you will see your logo appear in the e-mail template:
The easiest way to test your e-mail template is to use the Send Email function. Navigate to Sales | Orders, and select an existing order made through your Magento store, and click on the Send Email button at the top-right corner of the order details screen, as shown in the following screenshot:
A pop-up message will appear asking you to confirm this; click on OK. This will cause another order confirmation e-mail to be sent to the customer's e-mail address, allowing you to test changes to your store's e-mail templates.