And just in case Susi isn't sure how to resize it...
Open the photo in Preview
Under the Preview menu heading "Tools" go down to "Adjust Size..."
In the resulting dialog box make sure "pixels" is selected in the pulldown on the right and if the padlock icon is unlocked, click on it to lock it. (This links the width and height to stay in proportion.
Then whichever is bigger (Width or Height) I suggest putting in a value like 800. The other one will change automatically.
Click "OK" and save the image (Command-S or File > Save)