Your problem (as I understand it) is that you have a square frame (say 6"x6") that you want to print for. Just having a square jpeg as @dpollitt isn't enough, because if you print it at A4 some of the image will not fit.
OddPrints allows you to upload your photo and type in the size of your frame. You can then download a new jpeg ready for printing at a standard size.
For example, if your frame is 2"x2", it will generate an image ready for you to print at a standard 4"x6". The jpeg you download will be your image surrounded by a grey border. In this case, the image will constitute 1/2 the width and 1/3 of the length of the jpeg.
Disclaimer: I faced this problem so many times I wrote this website! I'm hoping this isn't too spammy because it's open source and free to use. Hopefully it will make more sense when you see it: