I've never used a pc for uploading pictures, but aren't you supposed to be able to just drag and drop them? At any rate, what I do is scroll down below the text entry box and select the "go advanced" button. When the new screen loads scroll down again until you see "manage attachments" and select that. When you click on the image selection line, on my phone or tablet at least, I get a box that allows me to select an app to use for locating the image file. Once you've selected the file, click on the upload button to actually attach it, the close the window and you should be returned to the original screen. Hope this helps!
It's not the size of the dog in the fight, it's the size of the fight in the dog. Mark Twain