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Thank you all for your help

I never knew Photoshop had a fit image function, I guess this is the way to go - although I have not tried it yet.

What I did find in the meantime is something that does it all for you

A program called Faststone Image Resizer (free):

In the advanced options there is an option to resize by "long side" (among many others).  This allows you to set the longest side for an image (height or width), and it will then batch resize yours images into any format you want, while also keeping the alpha channel.

This in combination with the "fit image" option in Photoshop should hopefully solve anybody else's issues when they search this topic out

EDIT: it would appear that I have too few posts to post links, so you will have to Google Faststone and select the image resizer product they offer if you want it.