We dislike having to tell clients that the photos they have sent us are not large enough to work for the intended use. Here’s a very basic guide that may help.
For typical press releases, a photo/file of 5” x 7” @ 300ppi is sufficient.
Photos for printing with stories in magazines, for creation of advertisements, or for brochures should be 300ppi at the size they will be used, or larger. Photography for trade show graphic panels should be as large as you can get them – and usually that’s not really enough resolution to adequately support the finished size. If you can manage 100ppi at the size used on the panel, that’s just right.
Files may be RGB .jpg files if saved at the highest quality setting. Your printer will want CMYK, but those don’t compress well and the file size is larger for the same image.
Photos downloaded from the web after a Google search are generally not of high enough quality to be used for anything more than a placeholder, and are usually copyrighted and cannot be used commercially without permission.