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Before you merge, open both of the images in a different viewer and have a look at the resolution and size of them both. You will want to try to match the resolution (in a perfect word both images have come from the same camera or the same scanner, and been saved in the same format. (jpeg vs tiff etc) You can try opening them in something like MacPaint, to look at those specifications. Steps for merging on the mac at the following link, be careful not to miss any steps, including the layer!
Let me know how it goes, and if you have any other questions feel free to ask! If nothing further, don't forget to rate and accept! :)