eBay has been modifying picture requirements to try to elevate the overall quality of pictures on the site. Please see https://ebay.custhelp.com/app/answers/detail/a_id/1489 for more details.
During submission to eBay, the user gets "Warning: 21916790 - Dimensions of the picture you uploaded are smaller than recommended. To reduce possible issues with picture display quality, eBay recommends that pictures you upload are 1000 pixels or larger on the longest side."
Your source pictures are not at least 1000 pixels on the longest side. One of the image modifications made by eBay was to force pictures to be at least 500 pixels on the largest side. This is currently the lowest size allowed and images smaller than this cannot be added. In addition, eBay goes on to recommend that pictures be at least 1600 pixels on the largest side. This is currently a recommendation and not a requirement. If, however, you list with images larger than 1000 pixels on the largest side, eBay will display a warning. The warning will not prohibit your items from being listed.
No solution is necessary. Your listings will still submit, you will just see the warning. Because of the move toward higher quality pictures though we would recommend that you consider creating all future images at least 1000 pixels wide with 1600 pixels being the optimal size.