Increasing memory allocated to PHP
[…] the WP_MEMORY_LIMIT option allows you to specify the maximum amount of memory that can be consumed by PHP. This setting may be necessary in the event you receive a message such as “Allowed memory size of xxxxxx bytes exhausted”.
This setting increases PHP Memory only for WordPress, not other applications. By default, WordPress will attempt to increase memory allocated to PHP to 32MB (code is at beginning of wp-settings.php), so the setting in wp-config.php should reflect something higher than 32MB.
Increase PHP Memory to 64MB
Increase PHP Memory to 96MB
No Image could be found at that URL
If you are confident that the image URL exists and is working then it may be a server configuration issue, you can confirm this by uploading the import file and checking the Preview column on the import options screen. If the Preview cell under the Featured Image or Product Gallery column shows a broken image then it is likely a server configuration issue best raised with your hosting provider.