Zalando is unusual among marketplaces: its images are portrait, not square, and the ratio is exact. Width to height must be 1:1.44, and Zalando's system rounds to two decimals, so a file at 1.445 becomes 1.45 and is marked wrong. A photo cropped by eye can miss that by a few pixels.
This tool reads your file in the browser and tests the ratio, the 762 x 1100 minimum, the JPEG requirement and the 20 MB ceiling. Nothing is uploaded anywhere.
The grey background you do not need to make
It is tempting to shoot on grey to match the packshots in the shop. Zalando's post-production guide says primary packshots must have a white background, and its image guidelines add that a light grey is applied automatically to white primaries. Coloured packshots from its background range are accepted only as additional views.
Model views work differently. Backshots and model views may use other settings, and a model view can serve as the catalogue view if its background sits in Zalando's colour range. This checker passes white or a light plain background, so read a fail on a lifestyle model image as a prompt to check the view type and not as a verdict.
Framing and rejection traps
For apparel and shoes, Zalando's grid puts the article at about 80% of the frame, centred both ways. Bags sit in the lower third, so the fill reading here is only a guide for them. Common traps in the guidelines include visible mannequins in mandatory views, non-English text on primary views, unrelated products in frame and duplicate files. Blurry images can be rejected even when the file size is correct. Electronics and toys also need one image showing the CE label.