When I switched to a new Theme, all my photos were too big or just didn’t look right. How do I adjust the photos for the new Theme so I don’t have to retake all the photos.
This is a very general question and hard to answer without knowing the details.
In general – check your theme settings and settings for each section. Most themes allow you to select “Natural proportions” or "Adapt to first image – this way your image will be shown without cropping.
If this is not quite what you’re after, forums have a lot of topics on how to re-shape your photos using CSS.
Look for solutions which apply to your theme as different themes require different changes.