Firstly, if you want the main page to just be a large image with text over it, set and use a static front page for your site. Static front pages do not show any of the date/category/tag stuff.
For the image, there are two ways to do it. One would be to add the text to the image before uploading it, but this is not good for search engines since they would not be able to read the text in the image.
The best way to do this would be to add the image as a "background" for the static front page and then you can write the text into the text editor as normal and it will show on top of the image.
The image you are using will be a little problematic since there is so much contrast (black to nearly white) and finding a text color that is going to work overall will be a major challenge.
And, another biggie for you. The image you are using is over 5600px wide and it is HUGE (file size=4.5MB) and that severely slows down your page loading time. That will hurt you with search engines since they penalize slow loading sites when it comes to page ranking. You need to optimize your images for the web (72dpi) and also size them to match the maximum image width for the posting area of the theme you are using.
Image file sizes should be kept under 150k for backgrounds and such, and under 100k for images put into the body of posts and pages.