I tried to find the form shown in your screenshot, on your website, but couldn't locate it.
Can you please share the link to a page where this "Upload Image" field can be seen?
As a general note, the styles from your theme or other plugins can be interfering with the default styles of that image upload field. You can use Chrome's inspect element tool, to check exactly which styles are applying to the specific page elements.
( ref: hidden link )