Oxygen Theme cropping photos

I’ll preface by saying I’ve spent hours looking for this topic, and found similar, but nothing quite exact, so I apologize if this is a duplicate.

I just launched my website today using the Oxygen theme, but the photo cropping is driving me mad.

If I have a photo to fit the “Featured” slider (750×380) when the user clicks on it to view the post, the image from the slider is cropped terribly when displaying as the main image at the top of a post often cutting off text within my photo.

I’m trying to create an image that will fit the slider – but not be cropped weirdly when viewing the article. I thought it would automatically make the correct adjustments but it’s not and I can’t seem to have one image look right in all places.

https://www.mintconditiondaily.com/ – Look at my “Conquering a 5K” photo in the slider, it fits there nicely. But click on that link to view the article, and some of the text from the image is cut off.

So for my “Why I Went Vegan” article, I used am image based on the post photo size (470×260) but now that photo is way too small for the slider.

Please tell me there is a way to get the same photo to work in all places.