I'm not aware of that feature being present.
If you do not have dedicated software, you can still do that in Windows Paint.
Go to Paint, find and open the pic you want. At the top left of the screen, click on "resize". In the window that pops up, make sure you click on the "Pixels" button, and that "maintain aspect ratio" is also checked. Enter a value in the "horizontal" box such as 600 or 800. Click "OK". Then go to the "save as", save as JPG, rename it slightly and hit "save". You now can attach it in TSB when logged in.