It can be tricky to capture the perfect image of your pet - especially as we may have what is a gorgeous photograph - but this isn't necessarily always the best choice for a reference photo to use for a portrait.
I will explain why in the samples below.
There's a few problems with this image - it is too far away to capture any kind of detail, and the sun is also altering how the colour of Todd's coat usually appears, this isn't a true representation of his colouring.
This image was taken in the evening and in a room which was far too dark to make it a good reference photo. The TV is also casting a blue tinge onto Todd's fur, altering his true colouring.
A lovely photo of Todd, but sadly just too far away to capture enough detail from the photo. This pose would be lovely if it were a closer shot of his face.
Overcast days are perfect for photographing for a portrait - you get the brightness required for a detailed photo, without the glare of the sunshine, which helps to capture the correct colouring.
This photo is a good quality close up - it is fabulous for capturing all of the little details on the nose and snoot area, all of those little white hairs will be able to be replicated within the portrait. This photo is also an accurate likeness of how the dog usually looks - it is best to chose a photo which shows your pets true character.
It helps to think of a pose which would look good on paper. Front facing isn't always the most flattering, especially for dog breeds with longer noses - looking up and side on photos often make for the most interesting portraits.