I agree with @Laura. Use PNG if you plan on using images with transparent backgrounds. If you plan on using high resolution images and resizing them, just be careful and test how they display on mobile devices.
No type is better than another, as long as Marketo accepts the file type. We typically use PNG or JPG for images. I'd just recommend keeping the file size on the smaller size if possible so you don't run into any issue there either.
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