How do people put those images in their posts?  I mean, I understand that the PBS-specific ones obviously come from PBS, e.g., "PBS Tour Guide" for example.  But what about the other ones, like, "Nerd and proud of it."  I tried to upload a picture from my computer but it didn't work.

Hey, Nora.  I'm too tired to do a detailed explaination and don't need to since another member (MannythePoolShark) has this tutorial in her notes:

Setting up your signature line.

The signature lines on the bottom of the forum posts are fun and an easy way to add more personality to your comments. They can also be used to advertise games and book bazaar deals or to highlight books you think are noteworthy.

1. First you need to make sure that any pictures you want to use match the requirements listed here: Valid file types: gif, jpg, png Maximum filesize: 20kb Maximum Dimensions: 75px x 550px To do that you will need to open up your My Pictures folder. Pass your cursor across the picture, it will list the dimensions, file type and size in a small box. If your picture does not meet the requirements then you can change it using a photo editing program.
2. Go you your account page.
3. Hover your mouse on the My account button on the top of the screen. This will bring down a drop menu, click the seventh link titled Account Settings.
4. Either click on the Forum Settings link or scroll down to the third box on the page titled Forum Settings.
5. To add a picture click on the browse button next to the Upload Image field. This will bring up a window for you to search through the pictures on your computer.
6. Find the picture that you want to use and click on it. Then click on the open button on the bottom right hand side of the window.
7. Click on the Update Forum Signature button on the bottom of the box.
8. When the page reloads scroll down to the Forum Settings box, make sure your picture is working.
9. To add text find the field titled Signature. Click inside the field and type the message that you want to show. Remember that this will be in every forum post you create.
10. When you have the message that you want typed in click on the Update Forum Signature button on the bottom of the box.
11. Again when the page reloads scroll down to check it.
12. Remember that your signature can only be 250 pixels tall including text. If you go over that limit it will still add the images and text but they will appear as cut off in the forums.
13. This is also the place to set up what information you want to show on your forum post. There are four check boxes between the pictures and the signature field: Show other members signatures, Show other members signature images, Show link to my book list, Show link to my Wish List. Check the box marked yes for the ones that you want to see and be seen in the forums.
14. When you have your signature the way you want it you can go into the forums and find one of your post. Check it to make sure it looks the way you want it to.

Note: You can find animated signature banners by searching the keyword blinkies. Several sites have ones that are already the right size. Please keep in mind this is a family site. Vulgar pictures and sayings are not permitted.

Public Profile Pictures

You may want to add a picture to your profile to showcase your personality. It is a simple process and makes posting in the forums a lot of fun. This is a step by step tutorial to show you how.

1. First you need to make sure that your picture matches the requirements listed here: Valid file types: gif, jpg, png Maximum filesize: 30kb Maximum Dimensions: 75px x 75px To do that you will need to open up your My Pictures folder. Pass your cursor across the picture, it will list the dimensions, file type and size in a small box. If your picture does not meet the requirements then you can change it using a photo editing program.
2. Go to your account page.
3. Click on the My Account button on the right hand side of the screen.
4. This will bring down a drop menu, click the first link titled Account Settings.
5. Either click on the Public Profile link at the top of the page or scroll down to the Public Profile box, it’s the second one on the page.
6. Click on the browse button next to the change icon field. This will bring up a window for you to search through the pictures on your computer.
7. Find the picture that you want to use and click on it. Then click on the open button on the bottom right hand side
8. Click the Update Profile Settings button at the bottom of the Public Profile box, you will now be redirected to your account page.
9. Go back into Account Settings to the Public Profile box to make sure that your picture is working. You can also check this by finding one of your posts in the forum.
10. If it doesn’t show right away try refreshing the page a time or two.

Note: You can find avatars that are generally sized right for this by searching the keyword avatar. There are several sites that have them in themes ranging from nature to humorous sayings. Please remember that this is a family site though. Vulgar pictures or sayings will not be permitted.

This is a link to a site to use to resize pics thanks to maydazee (kay m)

This is an additional tip for people who might have a picture you want to use for your public profile, but are still having problems with getting the picture sized correctly.  You can go to www.shrinkpictures.com to easily get your picture re-sized and centered correctly.  After arriving at this site, click on "Shrink Avatars".  It only takes a few seconds.  Just type in 75 x 75 in the pixel size box.