in an a!imageField() is there a way to remove the white border that is put around the image when it's put in the interface?
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Appian supports transparent pixels, so I think it comes down to what component the image is placed upon. I think if you have an interface with nothing but an image component, you'll see a white border, but if you filled your entire interface with a billboard of your chosen background color, and put the components in the overlay, that should show images without issue. If the image goes on a card and it has transparent pixels, that should work the same way too.
You could also cheat and update the BG of your images to match the background you'll be placing them on so you don't even realize there's a box displaying around it.