It's not any special program. You can find instructions for uploading images
here under the New Cats forum.
A lot of us here us photobucket.com. You need to sign up for an account, but it's free. Then you can upload pictures from your computer to photobucket. When you're posting a message, go to your photobucket page and under each picture you should see several boxes with different codes. Copy the IMG code, paste it into your message, and you should get a picture the same size as your original.
The problem with continuing to use attachments is that eventually you will max out the space you're allowed on TCS. Then you either have to stop adding pictures to your posts or you have to delete some images. When you delete an image in your attachments, that image disappears from all of your posts, forever.