Hey Micah, Welcome to TDP.

Flickr is one of the easiest places to post images from. Above your image is a row of buttons. Click on the last one (Share). Under 'Grab the HTML/BBCode', select BBCode, then choose a size. The forum's maximum width is around 800 pixels. Pick the largest size that has a width of, or under, 800. Copy the entire text that is in the edit box (directly above the size selection box). Paste into your post. If you hit 'preview' or 'go advanced' when editing your post you should see your image as it will appear to everyone.

It sounds like a lot of steps, but it's pretty simple and quick once you've done it a few times.