There might be a limitation on picture upload vs number of posts. Not sure.
I use my own server for pictures but you can easily setup a photobucket account for free and use it to post your pictures in any forum. Most forums won't allow you to upload large pictures because they consume quite a bit of bandwidth (when they are displayed to other users of the forum) and bandwidth can get very expensive.
Here is a tutorial on how to use Photobucket and how to post pictures in a Forum.
As far as the profile picture you have 2 limitations, one is size as far as pixels and the other is size as kb that it uses up in your hard drive.
For the Avatar (the image on the left side under your screen name) these are the values:
Your avatar must be no bigger than
200 pixels by 150 pixels in size.
Uploaded avatars from your computer must be no larger than
50 KB.
The following file types are allowed: gif,jpg,jpeg,png
For the profile picture:
This section will allow you to specify a photo to be used in your profile which is viewable by other board members.
All photos must be no larger than
50 kB.
Photos must be no bigger than
800 pixels by 600 pixels.
As you can see for both the limitation in kB is 50, however the actual size on the screen can go to 800x600 in the Profile photo and only 200x150 in the Avatar.
I hope this helps.