My 1st Question as member...

I do illustrations for print and web on PS elements 8. I never had an issue until yesterday:
I have 2 different illustrations, both saved as PNG. both 300dpi. both are 9x12 print size. Both RGB.
The issue is the both need to be under 10 MB with 300 dpi. for submission and illustration A is 8.5, but illustration B is 23 MB. What did I do? What should I have done, and how can I get illustration B under 10mb?
Thank you in advance and I am looking forward to meeting good folk here.
Can I add my website address here?