Saving your Image as a JPEG:
If your image is saved in a format other than JPEG (.tiff, .psd, .pict, etc.) it will not work. You can save your image as a .jpg in multiple ways. In Photoshop and most other image editing programs, you can go to File > Save for Web and Devices in the menu. Notice the box that allows you to set quality. In general, a higher quality setting (10 or 12) is better, so long as the size is less than 1.8MB (it should be significantly less). "Baseline Standard" is fine. In Photoshop, you can also select File > Save for Web & Devices, which will give you a preview of the image and control over various settings—just be sure to save it as .jpg, not .gif.
Preparing and Resizing Images: Overview
To resize your image so that it conforms to the 1920 pixels at the longest dimension protocol you will need photo-editing software that can resize images, change image resolution, create new images, and save images as JPEGs. The steps will vary depending on the photo editing program you are using; see Resources section for photo-editing software options. Below are generalized instructions for Photoshop: