add modifier to png image? Im following this tutorial here: https://www.youtube.com/watch?v=bF3O4qbaMqs
which explains how to curve and animate text. But I've designed my text in gimp and imported it as a png file into blender. The video goes on to use the object modifier tab which isn't available for the png file, what can I do?

Many thanks
As you have read from the other answers this is not a good way to do this. I would use actual 3D text like they did in the tutorial you posted. 

However to give your question an answer. 




Import image as planes (choose your text PNG)
Make sure you subdivide your text plane to give it more geometry to bend. 
Make an Empty and leave it in the center of your text. 
on your text/image object add the simple deform modifier and choose the empty as the object to bend around. 


Keep in mind you are very limited this way because you are using a picture of text and not actual text.