Figures sit straight on the white page. No frame, no rule, no shadow: the system defines none of those. Prefer a transparent PNG. Four widths ship, 5.2in, 5.6in, 6.3in, and 7.0in; pick the one that keeps the source image above roughly 190dpi at print scale, and never upscale past native pixels or re-encode the source. The marks below stand in for real figure content. The four width samples use a landscape mark on purpose: a portrait stand-in sizes by width and grows tall, so the four widths read as four heights instead.
Screenshots only. A UI screenshot is white to its own edge and has no edge of its own, so it takes a 0.5pt hairline. An illustration, a render, a map, a chart, or a transparent PNG does not. The mark above stands in for a screenshot; nothing else in the class ever gets a border.
Four widths, the figure row, and the screenshot exception. Captions 10.5pt quiet ink, centred, 6pt under the image.