https://imgur.com/t/pics (you can type /r/pics, but imgur changes it to /t/pics)
https://imgur.com/t/politics - for example
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But I like your thinking of pulling from multiple subs based on topic a lot.
Thanks for pointing the imgur link. I wasn't aware of it but it looks really interesting.
Imgur requires a title/description for posts-
Use imgur, select the subs you want to pull imgaes from, and search the description/title for the TOPIC keyword you are looking for... then display those, with a a hover that shows the subreddit the image came from.
https://imgur.com/t/pics (you can type /r/pics, but imgur changes it to /t/pics)
https://imgur.com/t/politics - for example
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But I like your thinking of pulling from multiple subs based on topic a lot.