Hi I've got a canon pixma mg6250 and whenever I print photos onto 4X6 canon photo paper there is always some of the image cut off the top, meaning I only get half of people's heads! Can anyone tell me what I'm doing wrong please? You have to tell the printer what size your are using.
Likewise, the photo edit software needles to know the size. There must be a print preview option where you can see what the output should look like before your print.

If the photo editor and the printer rate working together, you should only need to set the size in one place.

Wait! By chance did you mean you print directly from the printer?
In that case, you have to set the paper size in the printer.

Paper Sources to Load Paper - PIXMA MG6250
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REFERENCE
When printing, select the correct page size and media type. If you select the wrong page size or media type, the machine may feed paper from the wrong paper source or may not print with the proper print quality.
For details on how to load paper in each paper source, see below.
-> Refer to [ Loading Plain Paper in the Cassette ] within this webpage.
-> Refer to [ Loading Photo Paper in the Rear Tray ] within this webpage.
-> [ Loading Envelopes in the Rear Tray ]