How do I replace in Tumblr a blog title for an image?

Please help.
I have this blog in tumblr and I want to know how can i change the text title for the image…. mind you i do not know much about this. I installed firebug and now that I have to change the script in html… but I do not know how to do it.
My blog is thehappierproject.tumblr.com and I upload the image i want to use on imgur.com.

I was able to change the blog title for the image on the preview but I was not able to keep it so when I refresh the page I can see it.

Can someone please help me…

Thanks

Gabi