Using the jpg file format, the average jpg file size used on the forum is about 40KB, so each member has room for about 125 images using the jpg format.
Using the gif file format, the average gif file size used on this forum is about 90KB, so each member has room for about 56 images using the gif format.
Using a combination of file formats will of course change these numbers.
If you have previous post using files in your file area and delete those files the images will no longer be displayed in the old post.
Linking to files from other sites, not in your forum file area, does not effect the amount of file space you are allowed to use. Just remember if the off-site image is removed the image will no longer be displayed in your post.