The images from most digital cameras are too large to be suitable for uploading to the web-based image gallery and will need to be resized accordingly. There are various methods and programs available to accomplish this and one of the most widely used is Adobe Photoshop. Here is the simplest technique for using Photoshop (or Photoshop Elements) to resize your images:
Choose Image > Image Size.
The Image Size dialog box appears.
Make sure that the Constrain Proportions box is ticked
Make sure that the Resample Image option is ticked.
Make sure sampling is set to Bicubic.
Enter 1920 as the Width if your image is in landscape format ... or
Enter 1080 as the Height if your image is in portrait format.
Click OK to complete the resizing.

The size of the images you can upload is limited to 1920 pixels in width and/or 1080 pixels in height. If your image is in "Landscape" format (wider than it is high) then it must not be any wider than 1920 pixels. If your image is in "Portrait" format (higher than it is wide) then the height must be no greater than 1080 pixels. All images must be in .jpg format. The system will reject any file that does not have a .jpg file extension.

Remember that 1920 x 1080 means width x height. A common mistake is to make a portrait format image 1080 pixels wide and 1920 pixels high. That would be 1080 x 1920 ... and it would be rejected by the system as the height of 1920 pixels exceeds the allowable maximum of 1080 pixels. In addition, the size of the uploaded file must be no greater than 1MB.
Click "File" then "Save As". Save the image as a .jpg using the "quality" slider to select the best quality without exceeding the maximum file size of 1MB
You may optionally want to apply some Unsharp Mask before saving or you can try using "Bicubic Sharper" sampling instead of Bicubic.