Actually with Flickr, you SHOULD be able to do it quite easily.
Get a picture into flickr, for example:
http://www.flickr.com/photos/iancull/433645689/
Now click "all sizes" just above the picture.
On the new page, click the size you want (assuming you uploaded a pic big enough to have multiple sizes). For example, we get to this pic (medium):
http://www.flickr.com/photo_zoom.gne...3645689&size=m
Now, under the pic, it says "grab the photos url" with a box below it. Copy the contents of that box, then paste it in between the IMG.../IMG tags, like this:
{IMG]http://farm1.static.flickr.com/151/433645689_7b2c07f4a7.jpg{/IMG}
and what you get is:
