ShipScript has been an eBay Community volunteer since 2003, specializing in HTML, CSS, Scripts, Photos, Active Content, Technical Solutions, and online Seller Tools.
Some have had success with Google, and similar large cloud sharing services, as long as the folder is set to public, and as long as the correct direct URL is located (not the link that leads to a webpage view). Some sellers will set up a website for regular image uploading to eBay.
When images are transloaded from a website, eBay will resize the images as they are resaved to eBay's servers. However, hosting sites that are slow may have difficultly keeping up with eBay's hammering for pictures. If that happens, you will know the images should be downsized first so that the server can keep up delivery speeds to match eBay's requests. The handful that you are currently uploading should not be a problem.
By the way, if you get into large multi-variations, you may find this variation builder useful. It will create the master record and variation block for you. Also has a checkmark to open a PicURL editor.
https://www.isdntek.com/ebaytools/VariationBuilder.htm
Yes, if you successfully transload to eBay servers, the photos will still be saved on eBay if your server photos go away.
eBay does not have a notification system for items going out of stock. If you are using the out-of-stock control on your listings, the empty quantities within variations can be viewed in Seller Hub by clicking the (x variations) under the item title in the Active Listings grid, but that is not very efficient.
Another option would be to weekly download the Price and Quantity template from Seller Hub > Reports > Uploads. That template can be uploaded again with the missing quantities refilled.
