There are 2 ways you can go about this. Either use html/css to insert the image into the header, or hide the form title altogether by checking off the "Show form title" checkbox and use a "custom html" control, css, and a little html. We went the custom html route due to using bootstrap styling for the forms. If you want to use LF Forms themes, then html and some css is your best route. In the "Form description" field in the "Form Title" section add the following html:

<img src="[url to client logo]" class="client-logo" />
<img src="[url to your logo]" class="company-logo" />

In the "CSS and Javascript" tab, add the following css:

.client-logo {height: [number]px;float:right;margin:[optional]px;padding:[optional]px;}
.company-logo {height: [number]px: float:right; margin:[optional]px;padding-left:[at least some to separate the logos]px;}

You will need to play with the padding and margin values to get the effect your are after, but this should get you started.