There are a number of possibilities:
Problem: The file size of the image you are trying to upload is too big.
Solution: A possible solution is to either reduce the dimensions of the image to a maximum width of 600px (or 1200px to cater to retina/high pixels per inch screens) or save your images at a lower quality.
Problem: The file type of what you are trying to upload is not valid.
Solution: Save your image as .jpg or .png (when there is transparency).