Latest UK regulations
You can get a photo checking template at Post Offices. The template ensures that there is sufficient distance between the key facial reference points for passport office to obtain a facial biometric. Applicants are required to submit two identical photos. These must be produced from the same film or from the same electronic file capturing the digital image. Digital enhancement is not acceptable as this can result in the capture of an inaccurate biometric.

Size and paper
Photos must measure 45mm by 35mm
Heavy weight paper i.e. cardboard, is unacceptable
Photos must be printed on high quality, medium / low gloss or matt photographic paper. Photos produced at home are unlikely to be acceptable, as they will not meet the high standards delivered by professional equipment and may not allow us to capture an accurate biometric, leading to potential difficulties when traveling.
Corrugated paper is not acceptable.
Trademarks or other printing on the reverse is acceptable, provided it does not show through when the photo is scanned onto our system. This is most likely to occur where bold back print is used, and the image captured is very pale. Proper lighting and exposure are essential in order to capture the level of detail required for all customers, but this takes on added importance where back printed paper is used.
Photos must have a backing that allows the countersignatory to certify one of the photos and sign in ink, without smearing.

Glasses
Glasses can often show reflection or glare from flash, and the frames can obscure the facial features we need to capture. This can cause problems when trying to capture the biometric. We strongly recommend that customers wearing glasses are asked to remove them to ensure that we are able to obtain an accurate biometric. Whilst this may mean that their passport photo does not show them as they usually appear, their photo will still be a good likeness.
Customers with a visual impairment should also follow this guidance. Customers who wear dark, tinted or mirrored glasses for cosmetic reasons must remove them otherwise a biometric cannot be captured.

General photo requirements
Eyes must be open and clearly visible, with no flash reflections and no "red eye"
Facial expression must be neutral (not frowning nor smiling) with the mouth closed
Photos must show both edges of the face clearly
Photos must show a full front view of face and shoulders, squared to the camera
Photos must not show the subject looking over one shoulder (portrait style) or with head tilted to one side or backwards or forwards
The face and shoulder image must be centered in the photo
There must be no hair across the eyes
Photos must reflect/represent natural skin tone
Photos must be clear, sharp and in focus
Photos with shadows on the face or background are unacceptable
Photos must be taken with uniform lighting and not show shadows or flash reflection on the face and head
Photos must show a plain, uniform, light grey or cream background. 5 - 10% grey is recommended as a background.
Hats or head coverings are not permitted except when worn for religious reasons and only if the full facial features are clearly visible.
Black and white photos are not acceptable
Where very young children are being supported, ensure that hands are not visible. We recommend that infants who are unable to support themselves are laid on a light grey sheet and photographed from above.
Photos must not be manipulated, for example, by removing spots or softening lines, shadows, etc

Exceptions to the photo requirement policy
For children aged over 1 and under 6 years, the requirements for mouths to be closed and eyes looking at the camera are waived.
When photographing infants under the age of 12 months, we only require that the image shows a good likeness. The requirements on eyes open and mouth closed, for example, are waived. In practice, we have also waived the template check requirement for babies. As we still need a good likeness of the subject however, we have not mentioned this in our application pack guidance.
Some applicants, because of either physical or mental disabilities, will find it difficult to meet the requirements. We are able to therefore accept a customer's statement that they cannot meet the requirement because of disability. In these instances you need that as good an image as possible is captured.