How a photo is fitted to a marketplace size
Marketplace image resizer builds a new image at the size chosen in its list, one target size per sales channel. The canvas is first filled with the padding colour, white or light grey. The photo is then scaled by a single factor until it fits inside the canvas and is drawn in the centre. Width and height are scaled together, so the proportions stay as they were and nothing is cut.
Where the photo and the target differ in shape, the space left over shows the padding colour beside the photo or above and below it. A photo smaller than the target is enlarged, and the result line says so. The file is saved as JPG or PNG; where a channel's size note says JPG only, the format is fixed. Several photos can be dropped together, and each gets a card with the old and new pixel size, the file size, a preview and a download link.
When to resize, and when another tool fits
The resizer suits a photo that is right in content and only has the wrong pixel size or shape. It adds flat padding; it does not continue the scene. A photo whose background has the padding colour blends into it, while a photo taken in a room or on a table gets plain bands next to it. Extend frame is the Herofab tool for that case: it places the image in the middle of a larger canvas and is briefed to continue the background, the floor, the light and the shadows.
Enlarging a small photo here adds no detail; Upscale raises the resolution of an image by a factor of two or four. Where the old background should go altogether, Remove background delivers the product on a transparent, white or light grey ground. A file that is still too heavy belongs in the Image compressor, and a HEIC photo the browser cannot open goes through the HEIC to JPG converter first.
Reading the result and what the tool does not check
The line under each file name gives the pixel size before and after and the weight of the new file. The tool fits the whole photo, not the product inside it: it has no setting that finds the product, moves it or changes how much of the frame it fills, so a product that sits small in the photo sits small in the result.
The padding colour is laid down before the photo is drawn, so the clear areas of a cut-out take that colour as well, also when PNG is chosen. A correct pixel size meets one rule among several: the file-size limit, the background and the content of the image are judged by the sales channel, and its rule page is the last word. The Image size checker compares a file with the published size and format rules.