The images you downloaded are output by different cameras, scanners or other equipment. The default resolution on some of them could be lower than 300 dpi. This doesn't mean that the file is not usable. Before you download a file, first make sure you check the "Print size" in both English and metric systems for both web and print use. Use any graphical software to adjust the image resolution (if you are using Adobe Photoshop, make sure you uncheck the "resample image" option before changing the resolution of the file to 300dpi). Click here for a short tutorial for changing the resolution in Adobe Photoshop.