When editing a ShPt page, the Insert tab in the "ribbon" offers several possibilities for inserting a "Picture"or "Link": from Address (URL), from Computer (upload, for Image only) and from SharePoint (relative address). The latter is not always visible, albeit rather convenient, as it allows to browse the actual ShPt contents to establish a link in the current page.
This is a known restriction, widely documented in MS/ShPt social sites and manuals: The "from SharePoint" option only works when
If you do not want to use the Publishing mechanism for some reason on your page(s), then you may work around that restriction by using relative links (/mysite/SitePages/mypage.aspx rather than http://portal/mysite/SitePages/mypage.aspx). The relative links will be adapted by ShPt, when the target URL changes in a "normal" way (by renaming or moving with standard ShPt mechanisms).