Once I had printed my photo, and trimmed it to the size I wanted, I simply added borders to make up the size of my chosen cushion insert. I could then continue with my cushion construction as per normal. To add a little extra interest I inserted pompoms into the seam, replacing my usual piped edge. I used Riley Blake's (size small) pompoms, also purchased at my LQS.
There are a number of different products on the market that can achieve this result; this is just the product that was most easily available to me. I would recommend doing a tester print (with something you would still use in the likelihood that it works perfectly) just in case there are some minor adjustments that you need to make before making your own custom photo cushion.
Of course, you don't have to have a picture of your favourite TV series. It can be a family photo to commemorate a special occasion. Or a scanned image of your children's artwork for longevity. Whatever you choose to print, this is surely the easiest way I've found to DIY a Photo Cushion.
I do not claim ownership, copyright, or anything in anyway to BBC's Sherlock wallpaper used on this cushion.
Should you choose to use an image of your own favourite TV series, please note that most licensing policies do not allow for commercial use. Meaning that, while it may be fine to use these images on once-off items for personal use, it would not be wise to make items with these images to sell at markets, as this classifies as commercial use and can infringe on copyright etc.