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: 54 Megapixel
- 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.