What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Hello Robert, there's no size requirement per se, what's important is to use an image that will complement the size of your form. One good way to test things out is to use the Advanced Designer feature and upload various images until you find what works the best.
I'll be listing a few guides below that would really help.
Related guide: Form-Designer-Tutorial-Let-s-create-fantastic-forms
Logo and Cover: https://www.jotform.com/form-designer/tuts/episode7/
Background Image: https://www.jotform.com/form-designer/tuts/episode10/