If you have any photo editing programme on your computer, then open the photo in that and then save it as a jpeg file.
Alternatively, open it with MS Paint which comes with Windows and do the same.
An easy way of resizig any photo is to download the MS Power Toy, Image Resizer.
When installed on your computer, right clicking on the photo and selecting Resize Image will give you a number of options for the size of the photo you want.