When developing a web application - what are the downsides to using a template rather than having something custom made?

I've been down this road before (two custom CMS platforms built over the years), and to play devil's advocate here: do you really need to build a custom CMS? What problem are you up against that aren't being solved by existing solutions? If those problems aren't fundamental, how much time and money will be saved by creating plugins for an existing CMS? I know the temptation to start from scratch, but if this is something to be used in production, the todo list is staggering: you'll need user authentication, security on the front and back end, admin interfaces, plugin infrastructure for extensibility, theming (if this will be used for multiple clients), third party integrations — and that's all before you get into really annoying and notoriously finicky shit like media uploads and i18n. If it's for learning or a hobby, go nuts, but if you're planning to use this with clients, save yourself the heartache and late nights and use something established and open source. Join the community and help fix existing solutions' shortcomings — that's a bigger overall contribution to the world. Good luck!

I have worked on projects handling transfer of large image files across multiple vendors and being served on web for different targets (mobile, web etc.). I have also implemented cloud and DevOps solutions for high volume/scale of data and transactions. While I could tell you a list of technologies which can be used - the problem here is more of design and architecture. So I will start with that. Also a lot of decisions are based on more information and the use case - so I will need information on certain aspects of your application. 1) For searching an image users will enter search terms. You will have to associate the search terms/tags to actual images. While there are multiple ways to do this association - what is the link between a search term and image in your case. Do you have a separate database/service which will link given search terms to given set of images? 2) Since you are talking about handling millions of images - they will need to be partitioned logically or bucketized for optimum way to store and search. Is there a natural distribution? Do your users access all the images all over the world? 3) For any data storage related to images - we will need to go one detail further. While relational DB might serve a lot of needs, there might be use cases where considering other storages will be needed. Some questions which will lead to answers are - what kind of data per image will be stored? How frequently this data might change? How many new additions will happen on this archive? How fast would you want the updates to be visible to users/consumers? 4) What kind of traffic do you expect roughly? This will decide a lot design around auto scaling, load balancing and other aspects. 5) I am not a front end expert - but similar principles of scaling would apply to front end stack. Though I can not necessarily answer what technologies might fit your needs best.

Drupal is getting better & is one of the best options for open source content & ecommerce on the market.

I think BuiltWith will do this for you. Plug in the domain and it will tell you which technologies it is built on. Most of the well-known video sites are custom jobs, but I'm sure WP could do what you want.