If you don't already have a photo editor, I recommend Irfanview. It is free, and easy to use.

This forum says pictures attached have to be less than 60000 bytes (or 60 kbs) in size - so you need to get it below that.

Open your picture in Irfanview. Go to file >> save as. You will get a box that prompts you to name your picture. On the drop down thing under where you'd type the name (Save as type), choose JPG-JPEG.

On that same box is a button that says "options". Click on that, and you will get a slider thing for the amount of compression to use. You will have to play with compression to get it under 60 kbs. I ususally use from 60 - 70 compression. Once it is below 60 kbs, you should be able to upload it.