From the Model Mayhem FAQ:

What are the image requirements for the contests?
Unlike portfolio images, height and width values are not directly checked for the POTD - only the total pixel count is checked.

On both regular POTD and POTD18+, the following conditions are checked on every submitted image:

- Images must be .jpg or .png
- Image must be at least 7kb in size

The following conditions apply for regular POTD:

- Total pixel count must be 16 million or lower
- Image must be under 1500kb in size

In the case of POTD+18, it checks for the following:

-  Total pixel count must be 4 million or lower
- Image must be under 800kb in size

Please ensure your images meet these criteria before you upload!