It may be because you are using an Apple device, iPhone or laptop. If, at the end of the file names it says .heic, then they won't upload. That file type is only used by Apple and is not in common use anywhere else. You will need to change them to the much more common .jpg format. It's free to convert them on this website:
https://www.freeconvert.com/heic-to-jpg You can convert more than one file at a time.
If the file extensions are .jpg, .gif, or .png, please try uploading the images one at a time. That is, add one photo then click the save changes button before adding another photo.