This person is asking for BBCode (common on forums). BBCode calls for hyperlinks just like html does, there are just differences in the syntax.
So what you wanna do is go to the photo you want in Flickr and then click the "All Sizes" link above the picture.
Next, take a look at the "2. Grab the photo's URL:" text. It is giving you a link to the photo displayed. You want to display the square tiny version and you want that to link to the large version. All you need to know is how to change the size in the url.
So here's a picture link I get:
farm1.static.flickr.com/149/422762990_09956bb949_b.jpg
Now that's for the "Large" version version as the "b" at the end of the filename suggests B is for Big me thinks.
When I click for the cute little square version, I get this url from Flickr:
farm1.static.flickr.com/149/422762990_09956bb949_s.jpg
"s" is for square.
Now you know what you need to display the square and link to the large...
In BBCode, it looks like this:
http://farm1.static.flickr.com/149/422762990_09956bb949_s.jpg
