While both megapixels (MP) and megabytes (MB) refer to the size of digital objects (such as images of your art), they measure completely different aspects of an image.

Pixels are dimensional size.
MB are like the weight of the image file.

MegaPixels (pixels or px)

• measure the size of digital images, sensors and displays
• Pixels are the length or actual physical size of your image

MegaBytes (MB or Mb) 

• measure the size of digital files (such as photos, documents, etc).
• MB is how big overall the digital file is

An image might be 550px x 309px. This means it is 550 pixels wide by 309 pixels tall. The overall size of the file however, may be 1 MB. This is how much space it will take up on your computer.

Image Size is the dimension - length and width - of an image in digital format. This can be measured in pixels (px), the format we use, or inches etc.

Image Resolution, or dpi, is the amount of dots per square inch of an image when it is printed.

The difference between these two is that image size designates how large your image will be viewed on monitors, and resolution reflects printer quality.

See this FAQ:
Image Upload Size
for info on what size your images should be to upload to your FASO Artwork portfolio.

For more info please review the following articles in full. 

Excerpt from this article:
How many pixels are in a megabyte?

The number of pixels in one megabyte depends on the color mode of the picture. 

• 8-bit (256 color) picture, there are 1048576, or 1024 X 1024 pixels in one megabyte.
• 16-bit (65536 colors) picture, one megabyte contains 524288 (1024 X 512) pixels. 
• 24-bit RGB (16.7 million colors) picture, one megabyte has approximately 349920 (486 X 720) pixels. 
• 32-bit CYMK (16.7 million colors) picture, one megabyte has 262144 (512 X 512) pixels. 
• 48-bit picture, one megabyte has only 174960 (486 X 360) pixels.

Excerpt from this article:
Image Size vs File Size vs Pixel Dimensions vs Saved Size

Images have more than one file size, and I don't think anyone has come up with good terminology that distinguishes which size is being referenced at any time.

Some file sizes:

• The size of the file on your hard disk
• The size of the file open in your image editor
• The pixel dimensions of your image

Excerpt from this article:
Photo Files 101: All About Megapixels, Megabytes, and DPI

Alas, there's no direct way to correlate pixel size and file size. A 10-megapixel photo might "weigh" less than a megabyte on your hard drive. Or it might "weigh" as much as 6 megabytes. 

The file size depends on several factors, including the number of megapixels, the file format you're using (such as JPEG or RAW), and the amount of file compression used to save the photo, which is sometimes referred to as the quality setting.