Downsampling Images

I am interning for a beef company this summer as a Communications Intern, and design is one of my responsibilities. I created a series of Google ads (250 by 250 px and 300 by 600 px), and I'm having loads of issues with getting them to look right. Each of the ads has photos from the brand's photo library, and I downloaded the largest size of each just to be safe. 

I originally designed the ads in Indesign, and they looked great, but when I submitted the ads for approval, I was asked to optimize them to Google's file size specifications (they were originally like 15 mb and needed to be 150 kb). 

To fix this, I imported the ads into Illustrator and used the save for web feature to cut some metadata, but after sending them back for approval, I found out the dimensions were somehow off. I am thinking that I must have never resized the photos, which caused the ads' size to no longer be the correct dimensions?

I tried importing the pngs of the original Indesign ads into Photoshop, and sure enough they were huge. So I tried resizing the ads using Image>Image Size to scale them down, and tried  different interpolation methods, but none of them worked. Every time I scaled the ads down, they looked blurry and it's been driving me nuts. Just to be clear, I started with 300 ppi photos and exported them at 300 ppi, so I know resolution isn't the issue. I also tried reconstructing one of the ads from scratch, in Photoshop, and while the typography looked fine, the scaled-down photo was still blurry. If someone has a solution for this, I will be SO grateful.