Summarize this article:

Photoshop is the most popular image editor.  It requires a subscription from Adobe.  If you do not have a subscription to Photoshop, you can download and install GIMP for free.  It has similar features to Photoshop. Use the following steps to open and image in Photoshop or GIMP:  Click File in the upper-left corner. Click Open. Select an image. Click Open. If you want to edit the photo, do so before making a copy of the photo.  You'll also want to add "thumbnail" or something similar to the end of the filename. When you are ready, use the following steps to save a copy of the photo:  Click File  Click "Save As'. Type a name for the image next to "Filename". Click Save. If you want to make the image fit a certain shape, you can crop the image.  The crop tool has an icon that resembles two right angles forming a square. Using the following steps to crop the image:  Click the crop tool in the toolbar to the left. Click and drag over the section of the image you want to keep. Double-click inside the image. It's in the menu bar at the top of the screen. This is the option to resize the image. It's in the drop-down menu next to "Height" and "Width". Type this next to "Width" or "Height".  10% is a good image size for a large-sized thumbnail image.  The amount you want to reduce it may vary depending on the size of the image. Alternatively, you can select "Pixels" and type the exact dimensions in Pixels you want the image to be next to "Width" and "Height". This scales the image size down.  You may want to apply saturation to the thumbnail image  You can do so by adding a saturation adjustment layer in the Adjustment panel to the right in Photoshop, or by clicking the "Colors" drop-down menu at the top in GIMP. You may also want to apply a sharp filter.  You can do so by clicking the Filters menu at the top of both Photoshop and GIMP. Use the following steps to save the thumbnail image in both Photoshop and GIMP.  Click File. Click Save as (Photoshop) or Export as (GIMP). Select JPEG as the image using the drop-down menu next to "Format" in Photoshop, or below "Select File Type" in GIMP. Click Save (Photoshop) or Export (GIMP).

Summary:
Open Photoshop or GIMP. Open an image you want to reduce the size of. Save a copy of the image. Crop the image (optional). Click Image. Click Image size or Scale Image. Select "Percent". Type the percent you want to reduce the image size. Click Ok or Scale. Save the image.