Use File->Save for Web. Now, your settings will depend on what the site allows. On the right hand side of the Save for Web (SfW) dialog box you will see height and width, put 600 in for your longest side, and then click the Apply button below that. Then, on the upper right hand side of the SfW dialog you will see a small circle with an arrow, click on that and choose "Optimize to File Size" from the drop down. Another little dialog will pop up. Put 125 KB in the box and click okay. Then press the Okay (or Save) button on the SfW dialog. Your LO should now be small enough to upload.

-Trish