"Yet" when it means "even" can be used. In your example, I would take out the word "now" and just leave "I only have one photo on the site but I plan to post yet more". Having said that, you don't really need the word "yet". You simply plan to post more. Better would be:
I have one photo on the site. Tomorrow I will post more. Then the day after tomorrow, I will post yet more!
Your question tags are perfect.
Student or Learner