When you no longer received text email from spammers, this proof one thing. The filter system or software of your email providers have successfully filtered it out. For this reason, it has became a threat to spammers as their emails no longer passed through.
In order to fool the text base software filterating system, they need to think of a new way to spam you. This is when they turned every valid words into some kind of words that you can’t even understand and you don’t know what they are selling. These of course do fool the system and some managed to slip through. As the filter software catched up with them, their emails are blocked and thrash out again.
This when some smart idiots turned the entire text into graphics as the software cannot read the content of a picture.