How can I assign a picture for a name-only text sender
I know the types of messages you are talking about. I do not have HTC Sense, but I have had Touchwiz, LG (don't know what they call their version of Android), AOSP (CyanogenMod) and here is what I found and have been able to add a picture to these contacts this way.

When these message come in, they do in fact have a number. They have a Short Code. I get messages from T-Mobile every month from 456 to tell me they are going to take money from me. 

I added a contact, then set the number to be the Short code. Then I edit the contact as I see fit. Change the user profile image, add additional numbers, websites, etc.

  

122 &amp; 129 are message that are sent by the T-Mobile Visual Voicemail service