Ideally you should download it, make it "yours" and then upload it onto your own space. I don't think you can adjust the size of something that is on another's site, it's still "theirs". You don't have any control over it (or if it will "stay") otherwise. With that site, http://www.publicdomainpictures.net/ I would "join" and download/upload/use their tools if they have any or get a free or inexpensive photo editor/art program to use on your own computer before uploading back up to your space there so it would be the size you want when posted here.