Had to admit I was not sure what a HEIC file even was.
After some looking it does not appear that it is an issue with posting on Ag Talk it seems the format is just not compatible with Windows and is not supported by it.
Also appears many photo programs cannot convert them or view them either.
So unfortunately it looks like you need to convert them at your end to a format that Windows based machinery can view and handle.
Couple different ones listed in this article but I have no experience on them or whether these are safe to download and use either.
Hope you can get your pictures converted to share here.
I enjoy your pictures and input /stories
From the article.
What you can do is convert HEIC files to JPG using a free app (such as iMazing HEIC Converter or Apowersoft's HEIC to JPG converter) or an online converter such as HEIC to JPG.
Edit: I disabled the link in red as it tries to automatically start installing the program when clicked on without letting you find out about it first.