@Bisher, I use Steam's built-in screenshot function (press F12 in-game). They come out in jpg format and are between 80 and 180 kb in size. I then use SimHQ's attachment feature for posting. Perhaps a host site would be better. I'll do some research to see which site to use.
After the recent PhotoBucket melt down a host site would be the last thing I would consider.
SimHQ stores the images on their servers so you have less of a chance of the thread becoming useless in the future if you continue to upload directly to the forums.
PhotoBucket thread @ SimHQ:
http://simhq.com/forum/ubbthreads.php/topics/4366684t#Post4366684Wheels