Hello,
The global e-mail signature for Roundcube in EPESI can be set by logging in as Administrator, going to Menu->Administrator->Outgoing mail global signature.
There might be a little problem with inserting the image thou. The editor in there has more advanced features like inserting the image disabled - we're going to get that fixed in the next version.
In the meantime, you can try one of two possible solution. Both will require you to upload the Company Logo somewhere available through an url. One you have the image ready, either browse to tha image with any browser and select "Copy" (Don't copy the address/link, copy the image itself) and paste it (Ctrl+V) to the footer editor window. It will create image object in the editor, allowing you to adjust some other parameters with right-click.
Another option is to modify the modules/CRM/Roundcube/Roundcube_0.php, find the line:
$fck->setFCKProps('800','300',false);
and replace it with
$fck->setFCKProps('800','300',true);
Hope this helps!
Arek