You may think that as long as the quality is half decent, that any old snap will do to base your artwork on. That may be the case, but unless you are planning a memorial portrait where there may just be a single image you love, I'd suggest setting yourself up a little photo shoot.
Get your subject comfortable. That = relaxed. If you're photographing your pets, play with them, tire them out with a walk, get all their favourite toys out, get treats ready. This approach works well for kids and perhaps partners too. You want to spend some time snapping away. Get your subject to look at you, to connect with you. Keep in mind that what you send over to me is all I have to work from, so your photo needs to capture something about your subject worth hanging on your wall.
Think about how you want to portray your subject. What is their character. Do you want energy or sympathy. If you're planning a group portrait dn't forget that I can work from multiple images, so if you take lots of pictures and you just want a bit from one and a bit from another, send them all over with your description/instructions.
The other great way to take a portrait is the selfie. This is you showing the world how you want them to see you. They are generally funny and make fo very engaging portraits.
Keep in mind I am working from the pictures you send. If you don't really like the way you look in the photo you send, then chances are you won't like the portrait I create from it. If you'd like a certain amount of 'tweaking' to create an image you're happier with, then let me know and I'll do my best.