Question about Image sizes for themes

I’m sorry if this is posted in the wrong forum but….I’m trying to find the best route for uploading images to my posts.

Some background info: My website is close to 2 years old. It’s a tutorial based website so every post has 5-25 images. I changed themes one time prior and had to remake most of my images to fit the new theme content area better. Currently the max width of my images for a post can be 608 pixels. So because of that I made every image for the past year or so 608 pixels wide and optimized the compression for smaller file size before I uploaded them. I was recently told that I should upload much larger image sizes such as 2000 pixels wide and let wordpress scale them the first time they are displayed and then my cache plugin will show the cached, scaled version every time until the cache is refreshed.

Should I be uploading larger images and let wordpress scale them to fit the posts? That would eliminate the need for re-creating images every time I change template formats. But I’m concerned with page load time. If they are much larger images, even with caching would it affect page load time? Or should I just edit my images on the front end before they even get to wordpress like I have been?

Thanks for any and all clarification on this.