Struggling with photo size

Hey all –
Still pretty new to wordpress and site building, I’m currently struggling with sizing/scaling photos. My client wants her photos to not bleed (have to scroll down to see the whole photo) on her home page slide show. Most of the photos uploaded have dimensions of 4000px by 3200px or 3200px by 4000px, so i’ve been scaling the images down to the recommended dimensions of 940px to 752px in the wordpress “edit image” section and the photos are still too big.

Can anybody tell me how I can make these photos fit the slideshow? Do I have to scale them with a image modifier like a GIMP or photoshop to a smaller height and width and then upload them? (If so recommended dimensions?) Or do I have to crop the images before I upload them to the media library and apply the recommended dimensions from there?
Site: https://www.lccphotog.com
-Using the My Folio photography theme
-Thank you