I love a challenge.
Try this:
Open the original 600 dpi file.
Select > Save for web.
There ids a small drop-down button to the right where it says Settings.
Click on it and scroll down to 'Optimize to File Size'.
Set it to 100k.
Click OK and save it as it is.
This should give a jpg of the same pixel dimensions but at 72dpi.
It is still too big to post as an image file. So....
Zip the file and add this as an 'Attachment'
You will see the dialogue in the post a reply box.
Put in a file comment, click the Add Attachment button.
Then Submit.
I tried this with a base 3"x3" @ 600dpi, 7Mb psd file and got it down to 93k.
It should be enough for folk to play with, [saywhat] , I hope.
As long as there are not too many colours in the original.
The pic below will help you find your way.
Cheers, Al.