How can i add my own themes?

I can't find the option to add my own themes anywhere. Can you help?

If you want to add a theme to your form, here his how: https://www.jotform.com/help/184-How-to-use-your-JotForm-themes 

Let us know if you need more help.

I meant adding a random image as a theme 

Do you mean like adding a background image on your form? Here is how to do it: 

1) Click on the Design icon:

2) Once on the Designer, follow this video tutorial: https://www.jotform.com/form-designer/tuts/episode10/ 

Let us know if you need more help.