From the PicMonkey homepage, click the Create new button at the top of the page and select Computer. Choose the photo you want to edit from your computer.
When you open a photo this way in PicMonkey it will be "locked" to the background. To separate the photo from the background, open the Layers panel by clicking the 3 stacked squares icon in the bottom right, then clicking the padlock on the background layer.
Click on your photo layer to edit it with the Image palette. To rename your photo, click the existing file name (sometimes: "Untitled") on the top tool bar. You can also choose where to stash it in your Hub.
Note: All files autosave to your Hub's "Unsorted" folder by default.