The files need to be 512kb or less, and the total of all the (up to 4) pics in one post no more than 1024kb.
You can look at the files in your Windows Explorer and see the size in kilobytes there.
It's not only about size length and breadth but also about resolution, so cropping will help but you probably will also need to reduce the resolution.
I use Microsoft Office Picture Manager, Edit => Compress => for the Web. Seems to work, the files come up usually less than 100kb.
I don't use Photobucket so can't advise on how to do it there, sorry.
There is another way of doing it described in the topic on Forum Tips
linky. I've never tried the resizer myself but others seem to use it successfully.
Good luck!