Text and button overlay on large homepage image

Hi.

I have a large image under the main nav bar in homepage (1265 x 681 px)

I would like to create text and a button that lies over the image.

Some questions:

1. It seems that the caption text will only allow a max of 40px size – can I make it larger than this?
2. How can I control the position of this text?
3. How can I make a button over the graphic and position this where I want? I can only seem to put it below the graphic.

Many thanks :)