How to convert HEIC to JPG or JPEG - can't upload photos to eBay

After a recent iPhone update I suddenly couldn't use images taken on my phone for eBay listings after uploading them to my Mac at which point I noticed the files were .HEIC and not the usual photo formats. A quick google search showed that Apple had started to use a new format which uses less storage space. All good and well but I couldn't find a way to convert them for use on eBay.

If your photos won't upload and you can't find a converter then a quick workaround I found was to email the photos to myself through Gmail and they automatically become JPG images instead of HEIC. The app may work differently but when listing a car on eBay Motors you cannot use the app and therefore wouldn't be able to upload photos that way either.