In your marketplace, from the Admin panel in the Design tab, you can configure two logos.

The desktop logo will be shown to users with bigger screens, in the top left corner of your marketplace.

That logo should be 336x80 pixel size. Actual logo size is 168x40 and retina and high dpi displays will use the 336x80 version. The file should be a jpg, png or gif.

If the photo you upload isn't 336x80 pixel size, it will be resized automatically and taller images will be cut in the middle (top and bottom will be cropped).

To enjoy the best results, you should use the recommended size.

Consider for example this logo: It's size is 336 x 65

Even though the width is correct, then you add this logo to your marketplace it will look something like this:

Something similar would happen to images that have the right height but not the right width. The only way to prevent this is by adding and image with the recommended size or at least that has the same ratio of width x height.

This square logo will be shown when people bookmark your site. It will also be shown as an alternative to social media's logo if you don't add one.

This logo should be (at least) 600x600 pixel size. The file should be a jpg, png or gif.

If the photo you upload isn't 600x600 pixel size, it will be resized automatically and taller images will be cut in the middle (top and bottom will be cropped).
To enjoy the best results, you should use the recommended size.

You can use a basic image editor (like Paint) or some online tools like Cropp.me or Cut My Pic.

For the best visual design results, it is a good idea to hire a professional designer at some point, but their services can be expensive. If your budget is tight, you can use a service like Canva's Logo Maker, logogenie or logo.com to create a logo fast and affordably.