Summarize the following:
Go to the folder location of the photo that you want to use. Click the photo you want to compress to do so. It's in the upper-left corner of the screen. A drop-down menu will appear. This option is in the File drop-down menu. Selecting it prompts a pop-out menu. It's in the pop-out menu. Your photo will open in Preview. This is in the upper-left side of the screen. A drop-down menu will appear. It's in the File drop-down menu. Doing so opens a new window. In the "Name" text field near the top of the window, type in whatever title you want to use for your image. By default, the compressed image will have the same name as the uncompressed version of the image. Click the "Where" drop-down box, then click the folder into which you want to save your compressed image (e.g., Desktop). If the box to the right of the "Format" heading says anything other than JPEG, click the box, then click JPEG in the resulting drop-down menu. Click and drag the "Quality" slider to the left to lower the image's quality. It's in the bottom-right corner of the window. Your image will be copied, compressed, and saved to the folder you selected from the "Where" drop-down menu.

Summary:
Find the photo you want to compress. Select the photo. Click File. Select Open With. Click Preview. Click File. Click Export…. Enter a name. Select a save location. Change the image to JPEG format if necessary. Adjust the compression quality. Click Save.