Step 1: First you will need to determine which color you would like to use for your new submit button. Marketo provides a great variety of button colors which you can download here. You can also use or create your own image for your new submit button, but be aware, you will have to update the “background-position” in the code below to fit the size of your image.
Step 2: Assuming you have already created a landing page with a Marketo form, go to the Design Studio and upload the image that you have chosen for your submit button.
Step 3: After you upload your image, click its name to view the URL. Copy this URL into Notepad for future reference or onto your clipboard.
Step 4: Go back to your landing page and drag-on a Custom HTML block from the right-hand side.
Step 5: Copy and paste this code into the Custom HTML block.
Step 6: Where you see “(ImageURL)”, insert your image URL that you copied earlier so that your URL is within the brackets. Example: (http://yoururl.marketo.com/button-submit08/gif).
Step 7: Click Save and reapprove the landing page. Once you reapprove the landing page, you will only see the colored button on the live landing page.
Note: You may also want to save the image URL or the full code that you entered into the Custom HTML block for when you want to create other landing pages in the future.
Source: Marketo Community