Issue: IMG posting/Attachment

Hi
I don't know if this is really a bug or if I'm just to stupid to do it right, but I'm having problems with posting pictures in my posts.
The weird thing is: sometimes it works fine and sometimes I get an 'invalid URL' notice.
I noticed that sometimes when I click on the image button to add a pic, I first get offered the option to upload a pic directly from my computer. In that case, when I change the option to upload from website, the action fails ('invalid URL'). Other times I click on the image button and get straight to the 'upload from website' option and then everything works fine. Help me, please?

also, I posted some direct links to pictures earlier in a thread and for some reason one of them showed up as an attachment...why is that?

EDIT: I think I found out what to do to make the pic upload work. But I'm still confused about the attachment thing...