When you create a new form, it will be using a default theme (White theme) and doesn't set any logo for your form.
To use your own logo, you'll need to create a new theme for your form.
Creating a new theme
From your MachForm panel, click the Edit Themes menu to open the theme editor.
Uploading your logo
Click the Logo button and then click the circle A button.
Click Select File and choose the logo file you would like to use. Your file will be uploaded, and your logo will be displayed within the preview once completed.
The recommended logo width is 640px, while the height is whatever you like.
Saving the theme
Once you’ve completed uploading your logo, click the Save Theme button and give your theme a name.
Applying the theme to your form
Assign the theme to your form by going to Form Manager, select the form, and then click the Theme menu.
Select the name of the theme you would like to apply to the form and click the Save Changes button.
That's it. You've successfully changed the logo of your form. Below is an example of a form with custom logo: