Howard,
I don't know Imageshack, but the process for Flickr is a little more tedious than Photobucket. But then Photobucket (free) has bandwidth limits that you can blow through pretty quickly in a month.
After you have uploaded your picture to Flickr and it is in your Photostream
1. Click on the picture so it is alone on a black screen
2. Right click on the picture - you'll get a drop down of "View all sizes"
3. For GCA I usually click on the "Medium 800" size.
4. When that version is displayed, right click on it and then click on "copy image location".
5. Paste that into the [img] tags ion GCA. It'll display at 800. And, if you click on it, you'll go through to Flickr and you can see bigger versions if you uploaded a bigger version.
6. If you want to post another picture you have to go back to the image on the black screen and click the arrows on either side to find it. Or go back to your Photostream.
Annoyingly, all your pictures are in one Photosrtream unless you organize them into albums by course. You can't upload to a specific album. The albums just point to the pictures in the overall Photostream. There is only one instance of the picture.