Regarding to original question: there is no common standard to force mobile browsers to landscape mode. Maybe the idea is a bit wrong; trying to represent panoramic images on tiny non-landscape media
Wordpress has plenty of themes. I know some HTML/CSS/JS but idea of making own template that works on multiple platforms is too big. To make it really compatible on all browsers and platforms is a really big job and you have to somewhat almost prosessional to succeed in that.
Once you realize everything is being watched on tiny mobile screens, you might get depressed

Try to portray your photographs on a matchbox in two second life-span..
I have solved this by using wordpress+free theme, see here:
http://kuvau.tuu.fi/ .. but probably not going to post my panoramas in that format

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BTW: First time I got a bit larger audience outside instagram on my personal website. The latest photo has been watched 350 times, thanks to mastodon network. I think we can officially say there is an instagram alternative? :O