I have purchased a photoshop mock up and my placed images are showing blurry. The mock up file is 4000 x 2680 px 72dpi. I have played around with the size of the images being placed. It doesn't matter how big or how small the image is, they all look great in the smart object layer and then blurry once saved.
Everything I have Googled says it's the size of my images but I have been playing around with the sizes for hours and can't figure out what to is wrong....
Without actual file to check it is difficult to understand what is the problem from your description. Can you at least post some screenshots?
Have you read manual from mockup developer? What method you are using to replace Smart Object layer content?
One thing you can try is to match document resolution to file beeing placed resolution. Go to Image > Image Size (take a look at Info panel or status bar) to determine document resolution. Change resolution to file which you want to place if necessary then try again.
If you have smaller file of what is necessary with resolution 72 and you are trying to place it into document 300ppi then Photoshop will silently enlarge placed document 4 times to match document resolution. If file is enlarged on top of that you may see blurry result.
I am guessing that you placed the photograph on the booklet cover?
There just might be something going on here. The pinkish background is 4000 pixels wide. The book appears to be 3000 pixels wide when viewed below, so one page is 1500 pixels wide and the image a little less. Say 1200 pixels wide. Have you opened the image outside of this template to see what its pixel size is? It has to be 1200 pixels wide or larger. Is that the case? You can see that expanding the image to fit the template would make it blury?
When you place an image into an existing image, it comes in at whatever pixel size it happens to be. You can make it smaller, but you absolutely can't make it bigger to match the image you are placing into.
72dpi is meaningless, and dpi seems to confuse a lot of people. It's the size in pixels that counts. If the primary image is 2000 pixels wide and the placed image is 1000 pixels wide. it will show as being half the size of the original image. When looking for image elements to place in a composite, you need to find images that are large enough to match the original image. Is that making sense?