You'd need a desktop operating system, I dont know how to do it on mobile. Upload the image to imgur.com. Once the photo is uploaded right click it and select "copy image address". Use the insert image option here and post the copied image address. You cannot just post the url to the imgur page the contains the image, you need to copy image address.
If imgur doesn't like .HEIC files, open the image in Preview on your iMac. File -> Duplicate -> Save As a .jpeg and then upload the jpeg to Imgur.
If for future reference you don't want your iPhone saving things as .HEIC, go to Settings -> Camera -> Formats -> and select Most Compatible instead of High Efficiency.