There are several ways of adding images in your layout.master file, you can use regular html markup in which case you need to make the url for the image relative to the site root. If you want to re-use the images across skins you could place them in a common folder like /Data/myimages, or /Data/Sites/[SiteID]/myimages (if you want to keep them separated by sites in a multi site installation).
Alternatively, if the image is really just part of the skin and you want to be able to use an url relative to the skin folder so that if you copy the skin and use it in another site it will still work, then you could use our included control <portal:SkinFolderImage to add it to layout.master using an url relative to the skin folder. The below example shows how to reference an image that is directly in the skin folder:
<portal:SkinFolderImage ID="imgs1" runat="server" ImageFileName="bottomleft_circle.jpg" AlternateText=" " />
note that each instance of this control must have a unique id, if you put more than one of these in your layout.master file use a unique id for each one or it will cause an error and fail to load your layout.master file.