The most challenging task with painting from photo references is recreation the illusion of three dimensions in the painting, together with appropriate aerial and linear perspective. Knowing these principles and keeping them in mind while painting is very helpful indeed.
Hi Ro, I just came across the fantastic site 'Paint My Photo', thanks to Lori Ippolito's comment. Is this a well-kept secret here on The Complete Artist? You seem very involved there, and it is a great resource!
I'm going to add a link, just in case anyone reads this and wants to get there. Hope that's okay.
Your photos are great! Thanks for publishing them on the site! I especially like the Paris street scene - I am thinking about doing that. It has fantastic shadows. Most of my photos end up with mid day shadows and are not as dramatic as yours.