Jenny, the url you paste above goes tonthenupload photo screen. You then need to click on the button for finding the photo on your computer. Then navigate to the place on your computer where the file is stored. Select it then upload.
All that said, we should not be uploading images downloaded from ancestry.com. Ancestry.com's usage agreement disallows the copying of its digital images to other commercial sites. Wikitree, while free, is still a cmmercial site.