Make sure your images fulfill the following requirements:
File Type: JPEG
Color profile: sRGB
Min. Size: 1.500 px on the long side
Best Size: 4.500 px on the long side
Max. file size (resolution): 61 Megapixel
Max. file size (storage): 300mb
DPI: do not matter
No watermarks.
Note: The bigger the dimension of your images, the easier it is for you to submit to different calls for entries. There may be various requirements for different contests. Therefore, by uploading files in the above mentioned best size, the more opportunities will be open for you.