What is 500K or less in pixels?

[Mark]

Hi
I am trying to enter the FW competition on line, am constantly rejected when uploading image. Am told to "Please select a valid JPG image 500K or less in size"
I am in JPEG and reduced my image size several times and no go.

So how do I do it and what does "k" mean in pixel size or inches
Your help is appreciated
Thank you