Photo quality varies online from site to site. Many sites are deviously clever in using photography to hide a coins faults. Poor pictures are one reason I don't bid on EBay.
For important bids visit the PCGS site and see if they have a photo of the coin. Enter the slab id number in the verification box on their home page and if a photo is available it will pop up along with the coins auction history and registry set listing. If the coin slab has the little gold shield on it you know a picture will be available. I have declined or gone forward on bids based on the high definition pictures available on the PCGS site.
If a PCGS holder has the little gold shield you can access a suite of high quality photos (called TrueView) by entering in your browser http://images.pcgs.com/trueview/(coin id #). Example: http://images.pcgs.com/trueview/34115923. You can download these images to your computer. Note, some coins will have pictures available even without a little gold shield on the holder.
A scratchy holder makes for a scratchy picture. Who among us has not bid on a coin because they couldn’t be sure if the scratch was on the holder or the coin? Wait for a PCGS quarterly special and send those old scratched and cracked holders in to be replaced. Strongly consider having TrueView pictures taken when you reholder any coin. The pictures will automatically be associated with your coins in your online inventory. If you display your sets in a PCGS photo album one click will automatically update the album with all TrueView images available. TrueView high definition images are the gold standard.
Avoid using the people advertising coin photography on the internet. I’ve tried and the results were terrible. Later I realized my poor judgment in shipping my valuable coins across the country to someone I didn’t know. GreatCollections has great photos (albeit in the slab) and their vendor (BluCC Photos) offers the service to collectors. I would use them for photos of coins in the slab but generally prefer to let PCGS do it when the coins are out of the holder.