I have become a sponsor on a forum which allows me to post an avatar whose maximum size is 150 x 50 pixels, or 6.8 kb, whichever is smaller.

I have resized the picture which I wish to use to 150x50 pixels, but it still takes up over 12 kb, which the forum software will not accept.

I have looked at the avatars of other sponsors, and even with the maximum 150 x 50 pixels, they seem to be able to get their pictures under 6 kb (with no loss of resolution).

How do I achieve this ?

All advice gratefully received...