The roundcube icon cannot be changed (every user on the server shares the same instance of roundcube). You'd need to install your own copy on your account instead of using the provided one if you want to do that.
As for the spam folder, make sure you set up SPF and DKIM if you haven't done so already. For the SPF record, create a TXT record. Name should either be blank (if allowed) or the name of your domain if not. Value should be "v=spf1 include:tommy.heliohost.org ~all". For DKIM, you can turn it on in Plesk, apply it, and then click the (?) next to it to see the records to create. Once those are set up, its just a matter of domain reputation. Gmail will start trusting mail on its own in time assuming you don't send spam. Choosing "Not Spam" on the messages will help Gmail learn to put them in the inbox.
Also, be aware hosting accounts have a max sending limit of 50 messages per day to prevent bulk email from being sent. If you need this increased, let us know and we can assist with that (that process will require sample emails as well as answers to some questions about what you'll be sending, how you handle complaints, and why it's not spam). Note that we recommend you do not resell or otherwise let the general public use email on your domain. If we receive an abuse report for mail from your domain, you will be subject to suspension.