When using the shop import/export feature, you can manage images used when importing by clicking Manage images.
To upload images, click Upload images. Select one or more images (you can select multiple images by holding down Control, or Command on an Apple Mac). If the images are very large they will be resized in your web browser before uploading. Once the images have been uploaded they will be listed under Uploaded images.
Previously uploaded images are listed under Uploaded images. You can delete images by checking the checkboxes next to the appropriate image names (or clicking Select all images) and then clicking Delete selected images.