How can I change the resolution of a lot of images all at once?

1. If you have a lot of digital photos, all at 72 ppi, and you’d like to mass convert them to a higher resolution (you don’t want to print images at anything lower than 200 dpi if you can possibly avoid it), the first step is to be sure you have them all together in their own folder. You’ll be converting all files in that folder.
2. Open Elements, and choose File > Automate > Batch (Elements 1) or File > Batch Processing (Elements 2).
3. In the Batch dialog box, for Files to Convert, choose Folder.
4. Click the Source button and browse to the folder in which you placed the images that you want to convert.
5. For File Type, I would recommend picking .psd, but there are other choices in the drop down menu. (In Elements 2, you have multiple Quality choices for .jpg conversions.)
6. Check the Convert Image Size checkbox.
7. Leave both the Width and Height boxes blank. Enter the resolution that you want your images to change to.
8. Click the Destination button and browse to a folder where Elements will save your converted images. Note that if you are converting JPG images to PSD images, you can have the Source and Destination folder be the same. The files won’t overwrite each other because of the different format.
9. Click OK to convert your images.
10. Note; if you open both your original image, and your higher resolution images in Elements, they will appear to be the same size. It is the print resolution that you’ve changed. What you see on the monitor is strictly a matter of the number of pixels in the image, and since you have not changed the number of pixels, the two images will display exactly the same. A quick way to get an approximate idea of the print size is to select the zoom tool (bottom right corner of the toolbox) and click on the Print Size button.