If you're using Chrome, get the extension "Photo Uploader for Instagram" so you can upload images from your PC/mac. When your pop-up opens to upload a file, you need to pick All Files in the drop down menu in the bottom right corner of the pop-up instead of Custom files if you have .png files to upload. .jpg files will work automatically, but you HAVE to pick All Files for .pngs (the extension will say "(.jpg only)" but that can be bypassed with my method)
If you use Firefox, there may be other similar extensions to the one I use, but I don't know if the Chrome one will be available on FF too. And of course Internet Explorer/Microsoft Edge do not have extension capabilities