Unfortunately, getting your images to 300dpi is not as straightforward as changing the image in Photoshop. An image placed from the internet will be probably 72dpi which is way to low to print effectively, unless you are placing the image at 25% or smaller in Indesign.
What are their effective PPI? Look at the info at the bottom of the links panel when you have an image selected. As mentioned, changing the so called DPI means nothing. It is a couple bytes of information in the header of a file to aid applications in sizing them on a page when imported.
If the effective PPI is too low, that's pretty much how they will print.
- What is the effective resolution in InDesign? (look into the link panel.) (BTW resolution of an image is measured in PPI, not in DPI!)
- Is the link up to date?
- How do you proof it? Did you turn on the high quality display?
- What file type are you using?
- What is the scaling factor of the image in InDesign?
- How did you change the resolution in Photoshop? What was the first resolution, did you change the size of the image?