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 save for web – Go to File > Export > Save for Web (Legacy) to open the Save for Web dialogue box. Notice the box that allows you to set quality. In general, a higher quality setting (70 - 90) will be adequate, so long as the size is less than 1.8MB (it should be significantly less). Be sure that "sRGB" is checked on, and that you save as a Jpeg, not a gif.
Preparing and Resizing Images: Overview
To resize your image so that it conforms to the 1920 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: