If you are hosting your own picture, then you may want to consider enabling eBay to standardise the picture size dynamically when it displays on the item page. This can help your buyers to see your picture and your description text more easily. Both the width and height are adjusted to the original proportions.

You can standardise your picture by checking the box at the bottom of the Listing Designer page. Checking this box will automatically resize the picture you provide so that its largest side is 400 pixels when it displays on the item page.

Note that standardising your picture does not alter the original version you are hosting nor does it store a copy of the standard sized picture on eBay servers. The original version is loaded to the item page and resized dynamically by the buyer's browser.

If you would prefer your picture to be shown in its original size in your listing, then you should leave this box unchecked.