Quite easy when you know how - so here is what you do -
Find a picture you like and Right click on it - a box will appear and what you want is something called 'Properties' at the bottom of it.
Click on Properties and another box opens - this time you are looking for the 'Address' for the URL (it will start with http and be followed by other signs, letters and numbers.
I'm going to use this picture whose address is -
http://static.guim.co.uk/sys-images/Sport/Pix/pictures/2010/1/17/1263725969077/Owen-Coyle-001.jpgSimply copy this 'Address' - please note that sometimes not ALL of the address is visable on screen, so take particular care that you copy all of it (including any 'hidden' part there may be) - in fact I usually scroll to the end of the address and copy back to the http - to make sure I get it all.
Once you have copied it - and are ready to post it, click on the icon (third from the left) on the bottom row of the boxes - on the 'create a post' bit of this site (the one just above the smiley wearing the sunglasses - this one -

When you do that you will get this thing on your post -
[img]http://[/img]
Simply then post the address that you have copied 'in-between' the two brackets and your picture will appear when you post like this -

If you are not understanding what I am trying to say - I suggest you click on the 'quote' part of this post and then you can see for yourself how I've put everything together to get the picture.
Once you get into the hang of posting pictures I suggest that you always preview your post before you actually post it up because not every photo will work - and that is why you sometimes get the little box with a red cross in it appearing.
Sometimes that is because you have not copied the FULL address across but more often than not it is because the picture as been copyrighted or something - and preventing you or anyone else from using it.
Hope people can understand my explanation.