You could try posting in the Community > Jobs section, but I'm going to be honest and say that $50 probably won't get you very far when you want a new design, sliced and coded into a template.

You could also try reading through some how-to docs here and watching some tutorials on creating themes, but you'd still need some experience with image editing and HTML programming.

When working with budget constraints... try not to re-invent the wheel ;)
Take a look at the Marketplace on this site and see if something like this will accomplish what you need... http://www.concrete5.org/marketplace/addons/backstretch/...

it allows you to use your image as a screen background.
Or
select a theme that has a similar feature built in (like Clearview?)
Or
Select a theme you like and change the static background image that comes with the theme. (We are currently doing this for a friend... compare the theme "Grunge Columns" to the site http://www.raftreeband.com )

Concrete 5 is Very flexible in allowing customization without starting from scratch.