The Open Tender platform is optimized for horizontal images. However, square images will also work. When you're editing photos, the width is what you should pay the most attention to in the sizes, as listed below. You should be uploading both different image sizes in order to optimize the user experience. To learn more about Photoshop exporting, scroll down to watch a step-by-video.
The optimal image sizes are:
Small: 900 pixels x 600 pixels, 50-75 KB file size
Large: 1800 pixels x 1200 pixels, 100 - 150 KB file size
To export images from Photoshop:
Click on File > Export > Save for Web
Select JPEG for image type
Set Quality to 25
Set the width to 1800 pixels for large images and 900 pixels for small images
Click on Save and select a location to save your image
For naming conventions, do: menuitemname_widthxheight
For example, for a menu item named Brussels Sprouts, the image name would be: brusselssprouts_1800x1121