Your question is so broad that my answer has to be as well. Start by making a working prototype. You can probably finish the software without any custom hardw

Your question is so broad that my answer has to be as well. Start by making a working prototype. You can probably finish the software without any custom hardware, at least the non-hw-specific parts (which is most of it in this case). Then when it's working, start thinking about making it into dedicated hardware.. Find out what minimum specs you can make your software for (if you're just one dude, consider something like an ESP, that should be able to drive the display just fine). Then you need to ballpark what BOM price is. If an OEM has a similar enough product, probably photo-frame manufacture already has, and your product can be more or less reduced to either some custom firmware for one of their models that has wifi. Alternatively, consider if what you want to build already exists.. Something like an Airtame or Chromecast app could be paired with any monitor in the world, you pick a few models and sell together with your software ? actually.. what are you even asking? Get some idea what the initial costs will be, contact investors, form a company, throw other peoples money into the idea.. Honestly, it's not for me, but some people just love doing that, and they're awesome at it.