Flickr is being a pain. There is probably some manner of nagging advert blocking the usual 'share photo' interface. After uploading your pictures and refreshing the gallery, click the picture you want the code for as usual. On that black screen on which you see your picture and the menu, right-click and select 'show page source'. In the results will be bit you want, in all the sizes. o.jpg is for full size, z.jpg is for the medium 640 pixel size. Use your browser to find the z.jpg in the page: click "view" and select "find in page", enter z.jpg in the box, click next. The first hit will not only give you the z.jpg URL but also both the height and width of that resize. You can use that to swap parts in an old embed code, or put together the whole URL ending in z.jpg and just use the img src="Flickr URL z.jpg" HTML string.