Some examples of image size:
640x480 displays the full image, and won't make a thumbnail:
1024x768 displays the full image, and won't make a thumbnail. Any size larger will thumbnail:
1920x1080 is the largest allowable size, and is displayed as a thumbnail at 1024x768. Users can click the image to view full resolution. Please message Maddiedog if you have any questions about posting images.
Here's how you upload and display images:
1. Click Post Reply at the bottom of the thread you're replying to. Below the post window that pops up, you should see a section that says "Filename:" and has a Browse button. Click the Browse button, as shown below.
2. On your computer, find the file you want to upload, and double-click it, or select it and hit the Open button in the bottom-right.
3. Press the Add the File button:
4. Your image will automatically upload. You should see a spinning Yin and Yang in a pop-up window while the file transfer goes through:
5. Place the image inline by selecting the file from the Attachments dropdown that has appeared below the post window, and clicking the Place inline button:
6. Move the BBCode that is placed into your post wherever you want your image:
7. Finish typing your message, and click the Submit button to post your reply with attachments.
Images uploaded to the site can be a maximum of 1920x1080 pixels, but it is strongly advised to keep them under 1024x768 to ensure users with smaller screens do not have to scroll.
Hope this helps.

Note: edited by Maddiedog on July 26, 2013 to add image size clarification at top