Update: Summer15 now supports images in emails. However it only works currently in case feed and in the "click here to view HTML version" link on the email record.

Your issue isn't with the HTML of the email, it is the embedded images. It is a long known 'limitation' of salesforce.

https://success.salesforce.com/ideaView?id=08730000000Gp9dAAC

If you need a hacky workaround, the image place holders in the email message body can be matched to image attachments based on an Id order.

Expanded explanation: (please don't do this)

The place holders come in two forms

• cid:imageName.png@asdf
• cid:asdf

If the placeholder is in the first form, then you can just match the attachment by name. If it is in the 2nd form, the place older position is the same as the order the image attachment was inserted

List<Attachment> attachments = [
    SELECT Id
        , Body
        , ContentType
        , Name 
    FROM Attachment
    WHERE ParentId = :email.Id
    and contentType like 'image/%'
    order by Id asc];

Map<String, Attachment> attachmentMap = new Map<String, Attachment>();
for (Attachment a : attachments){
    attachmentMap.put(a.Name, a);
}

//list all the content-ids (cids) in the email
List<String> cidList = ....
for (Integer i = 0; i < cidList.size() && i < attachments.size(); i++){
    String cid = cidList[i];
    if (cid is of first form){
        .... get attachment by name
        Attachment a = attachmentMap.get(imageName);
    } else {
        .... cid is of 2nd form
        Attachment a = attachments[i];
    }

}

Limitation

Sadly, there is no apex email support from platform events. Per the documentation:

Sending an email message from a platform event trigger using the Messaging.SingleEmailMessage class is not supported. The email can’t be sent because the sender is the Automated Process entity, which has no email address.

Workaround

You can, however, send emails via email alert where the From Email Address is NOT "Current User's Email Address" but rather some other organization-wide email or default workflow user.

The reason, as outlined in this answer, "emails sent by the Automated Process user was "autoproc@YOURORGID" with no ".com" appended. This meant that Gmail would ignore it and the email wasn't being delivered."

In Apex, you might be able to use the SingleEmailMessage.setOrgWideEmailAddressId method to set the org-wide email and thereby the email be sent from a user other than the automated process user.